I had the same problem with a sim mayor and a showbiz cat leaving for work at the same time. The helicopter turned up and I watched the cat jump onto followed by the sim. Thinking they were both safely on the way to work I switched focus to something else only to later find both the mayor and the cat wandering round the lot. As the sim in question was a fairly crumbly elder I simply retired them to get around the problem. Having the 'helicoptered' sim use their own car should get around the problem and I will bear that in mind for the future, thanks.
I'm also pretty sure that sims (always) share the car pool when a sim arrives as another departs for work... I still find it very funny watching the criminal mastermind going to work in the police car.