If anyone has "Lost" their sims tombstones ie: Sim dies and no tombstone or urn to be found on lot - try going to the cemetary and clicking on the mausoleum and then select "Manage the dead" option, your Sim will enter the building and a window will open up containing the tombstones of all the "unclaimed" dead, see if your dead sim is there. 

For those of you who think that EA's use of fences or rails not available in the game might be the cause of the corrupted lots you may be interested to know that these very objects have now been extracted from the game and are available at MTS along with some recolours, click on link  http://www.modthesims.info/download.php?t=372486
Personally I still believe the lots were corrupted in other ways but I have an open mind and would be interested to hear if installing these cures anyones riddler tiles, I have installed them myself but not yet tested them out as a potential cure.

I transferred the Goth lot into Riverview AFTER having replaced all the riddler tiles which appeared during play in Sunset Valley, riddler tiles re-appeared almost immediately and continued to appear even after cleaning them up yet again in Riverview, I have since then bulldozed the EA lots and replaced them with my own lots (in Sunset Valley AND Riverview) and have had no further problems. I think this is the only way to purge the Riddler tiles completely.

I have tried numerous ways to salvage my Goth lot infected by ?s as I had spent a lot of time on a complete re-design, I have now come to the conclusion that this is an Impossible Mission! Nothing I have done has resulted in a permanent fix and I am now of the belief that the base lots, as shipped by EA, are corrupted and, whether you have AM installed or not, CC of amy kind installed or not or have never even played one of the affected lots or not, eventually the Riddler tiles WILL appear.

I have established the following facts about these lots while trying to salvage my own:

1) ALL wall and floor tiles on the lot become unselectable by the 'design tool'
2) Copying any remaining wall patterns or applying new patterns to affected areas is possible but only works while you stay on the lot
3) Repairing the lot by re-applying wall and floor coverings from scratch (ie: not copying and pasting from remaining walls) will return the design tool functionality BUT again only while you stay on the lot
4) Saving a lot to the library (with or without a familly) after it has been repaired as above may reduce but not eliminate Riddler infestation
5) Transferring a repaired and saved lot from Sunset Valley to Riverview has no effect, riddler tiles may be reduced but not eliminated
6) Repairing a transferred lot in Riverview has no effect, once you leave the lot the tiles will return
7) The Art Gallery has terrain under the foundations which is covered by floor tiles but after deleting the tiles a flat grey surface is left which cannot be modified or deleted
The Wolff house has no floor tiles below the foundations as the terrain is already covered by the same flat grey surface found under tiles at the Gallery
9) The Goth house does not seem to have this problem as there is only clean terrain below the foundations, but if you apply floor tiles and then delete them the same flat grey area appears
10) Although only some of the terrain below the Gallery appears to be affected applying floor tiles and then deleting them shows that, as with the Goth and Wolff houses, all terrain underneath a foundation is the same
11) All 3 lots reported as having Riddler Wall infestation also have Grey Terrain blight
12) The Grey terrain problem is linked to the house and not the lot, replace the house on any of these lots and the problem disappears, if the Grey terrain problem has gone so, I assume, will the Riddler tile problem have been cured
13) Grey terrain cannot be deleted even using the sledge hammer tool nor can it be modified using the terrain tool, it can be covered up but not eliminated
14) One other possible symptom of Riddler tile infestation I have seen is that ghosts lose their colouring and glow more brightly, their thumbnails assume an appearance like that of a photo negative (black face & white hair etc:)

As far as I can see there is only one possible cure for Riddler Tile infestation and that is to replace the three affected lots with duplicate lots not of EA origin each time a new game is started, any of you builders out there up to a challenge?

I am off to try an an experiment to see if replacing all the graves from an affected Goth house with graves from one not yet affected will cure the ghost problem. My Riverview Goths finally moved out of Goth Manor taking their dead relatives with them, they moved into another haunted house that came with the Riverview neighborhood and the already resident ghosts are fine but living alongside the overlybright, monochrome mutants imported by the Goths (which at least tells me it's not a graphics card problem)

I just got infected with the 'Question Mark' walls for a second time, having read all the posts no-one seems to have yet come up with a solution so I decided to investigate the problem in a bit more depth myself and, although I have no idea what caused it or how to stop it happening again, I believe I have found a way to fix the corrupted lots - permanently (well, at least until the next time)

I was playing the 'Goth' lot when the game crashed whilst I was trying to access CAS via the 'Dresser' cheat, when I loaded the saved game most of my beautiful walls and floors were gone, I checked the 'Art Gallery' lot as it was this one that I had had problems with the first time I had 'Qustion Mark' disease and several of you mentioned this particular lot in your posts, I was not surprised to see that there was not a single wall or floor that hadn't been corrupted. I then checked the 'Wolf' lot which I hadn't played but was curious to see if it too had been given a new paint job after reading that this problem appears to only affect certain lots, sure enough, most of the exterior walls and several of the interior walls and floors were now a horrible shade of blue and red. After investigating all three lots I discovered the following problems:

1) The deign tool would no longer work on any wall or floor on the lot regardless of whether it had it's original pattern or the 'Qustion Mark' pattern
2) The eye-dropper tool did still work and not only could any remaining wall or floor designs on the lot be copied and pasted - so could the 'Qustion Mark' pattern (in case any-one out there REALLY likes it that much)
3) When selecting a pattern to copy with the eye-dropper the game should automatically highlight the appropriate thumbnail in the catalogue but this doesn't happen, in fact, it doesn't even show the right category anymore.
4) Although the eye-dropper tool will copy and paste any remaining patterns so you can cover up the 'Question Marks' it does not restore any functionality, the ?s have gone but the design tool will still not work.
5) If you use the eye-dropper tool to select a wall or floor which has had the pattern replaced by ? you will still get a tool tip telling you the name of the base pattern used (useful if you can't remember what was there before)
6) This problem affects every wall and floor on the lot, it makes no difference whether it is an unchanged base game design, your own unique concotion (saved or otherwise) or a download from the exchange.
7) As far as I could see only three lots are affected by the 'Question Mark' disease when it strikes:- Goth Manor, Lone Wolf Manor and Sunset Institute of Modern Art. (however I have not had time to check all lots yet)
8 The Sims 3 save game as well as its backup are affected and deleting cache files makes no difference at all, new games are not affected and nor are any other saved games

Fortunately the lots can be fixed but there is good news and there is bad news, first the good news:

If you are prepared to restore the lot to its original, unmodified, Maxis designed state, either because you haven't changed anything anyway or because you are more interested in saving the familly rather than their home then the fix is quick and fairly simple.
1) Start a new game and after selecting any familly EXCEPT Goth or Wolf go into 'Edit Town'
2) Click on either of the above families and in the pop up window select EVICT (option furthest to the left) leave belongings behind.
3) When the lot is empty click on it again and this time select SAVE COPY TO LIBRARY (just click OK on any warnings about interractions being cancelled)
4) Click on the evicted familly and get them to buy their house back again and then just do the whole thing again for the other familly
5) Save the Art Gallery the same way as the other lots (just no familly to evict first)
6) Click on options to return to MAIN MENU
7) Select your damaged game in the normal way and then go into EDIT TOWN
Change your active household if necessary and then EVICT the familly from the affected lot with or without furniture but make sure any irreplaceable rewards etc: are either in your Sims inventory or your household inventory
9) Demolish the affected lot with the 'Bulldozer tool' and then replace it with the saved one
10) Move the familly back into their new 'Question Mark' free house (repeat as necessary)

Now the bad news (and this is what I had to do)

If you have done a lot of customising of the lot and want to salvage as much as you can you are going to have to replace EVERY single wall and floor on the lot! If you just copy any of the original walls that are left and paste them over the ?s it won't work, as soon as you leave the lot or the game the ?s will return and probably in greater numbers!
1) Select the design tool and waft it over the lot to verify you need to do this, if buy and build mode objects 'light up' as you pass the tool over them but walls and floors don't then you're out of luck - read on!
2) Remember, do NOT copy any affected walls or floors with the eye-dropper, if they are not reacting to the design tool they are useless, instead, use the eye-dropper tool tip to identify the base game design used (not always obvious in-game if replaced by a ?)
3) Select the base wall or floor used from the relevant catalogue selection and paste it into the lot
4) Use any existing walls or floors as a guide to reproducing the design you want, don't forget that you may have customised objects in the room with elements from the wall or floor you are trying to recreate so use the design tool on the object and copy the pattern or colour back into your design
5) Once you have recreated your design on this new section of wall or floor then copy it with the eye-dropper tool and replace ALL the relevant sections on the lot, even if the original pattern hasn't been replaced by a ? it still needs to be replaced, and, just to rub salt in the wounds, you will have to pay for everything all over again, it's as if you are decorating bare walls!
6) When you think you have replaced all the corrupted areas use the design tool again to make sure you didn't miss any bits by repeating step '1' above. Waft the tool over the lot and you should now see that the replaced sections of wall or floor will 'Light up' as the tool passes over them, if you did miss a bit use the design tool first to make sure that the section you copy has been repaired.

I hope this will help some of you fix your games and also help someone much smarter than me to identify the cause of 'Question Mark' disease, by the way, I have the latest version of AM installed, my game is patched to 1.3 (still don't feel confident enough to re-install 1.4 after what it did to my game the first time) I have far too much custom content but can't seem to control my downloading addiction and my computer would be described as High End so, given that this problem has been encountered by others who don't use AM, have patched and unpatched games, no CC and numerous PC configurations, I think it must be something EA did - again!

Oh dear, this fix did work, but not for very long! Should have done a bit more play testing before posting. However I do have more information that may give someone more insight into what is causing this problem.
After replacing all of the corrupted walls and floors on the Goth lot I had no further problems and, in case the original save game had bad data, I did a 'Save as' and re-named it to something different so I got a completely seperate save game. When I re-started Sims 3 I loaded in this new saved game and the ?s were back! I re-installed the game and re-patched it to 1.3 in case any of the game files had been corrupted and tried again with the same saved game, the problem was still there so I deleted all of the wall and floor coverings that I could and replaced any floor tiles that I couldn't delete with an unmodified pattern from the catalogue, I then saved the lot to the library as both an occupied lot and as an empty lot.
I went back to the main menu and started a new game and I deleted the Goth lot and replaced it with my saved empty lot, there were no ?s this time but all of the new floors were again undesignable. I replaced all the floor and wall coverings for the third time and then saved the lot back to the library and moved the Goth family in, I noticed two single floor tiles with ?s but everything else was still OK so I replaced those and carried on playing without problems. After an hour or so I went to the main menu and checked out a couple of lots on an old Riverview save, then I went back to my earlier Sunset Vallry game only to find the Goth lot was once more covered in ?s.
I am going to try moving the Goth lot into a Riverview game to see if this makes any difference but I am beginning to suspect that this ? disease may be incurable once a lot is infected, sadly I may have to demolish the lot and the Goths will have to move in with the Crumplebottoms, they won't like that!