I remember reading about people having trouble finding all of the poltergeists in the ghost hunter career. Today I was playing and was having a heck of a time finding the last of 3. I slowly scanned the lot, noticing that the woooo sounds got louder in a particular corner, but nothing was wobbling. So I started clicking on every object I saw and found that a large hay stack needed exercising. The stack was too large to see the wispy animations and for whatever reason it did not lift off the ground as other objects do. I hope this helps anyone having similar problems finding the last poltergeist to exercise. Just find where the sound seems loudest and click on every object in sight.
Or if you stop controlling your Sims, they will randomly fetch the ghosts themselves. I know that the ambitious trait has a different influence on the firefighter and ghost hunter careers, I think it's due to the fact that those are the only 2 jobs with "mini-games" incorporated. Where you have to "solve the problem" yourself.