I've driven around with dead passengers before in the car.
Ok, now you sound creepy j/k
Seriously though, there seem to be several *glitches* in the game. I remember being involved in a shootout w/ an angry driver and of course peds running, drivers all trying to ram everything and get out of there. One ped was hit by one of their fleeing cars and was dead but he was *stuck* in the air.
This isnt creepy but glitchy at the grove street garage. I park a car outside, go in and take one out, park it, get the other one and park it inside, as soon as I go back outside, that car I took out, is gone, every time.
I also pulled up (just today) to put a car in there. There was a van already sitting in one of the parking spaces w/ the driver's door open. I parked in a space, got out, (van still in view) started to walk towards the garage, spun the camera back around and the van had disappeared that quick.
I even came back to Franklin's house after a mission and Franklin's (modded) car was sitting outside of the garage facing the street when I parked it facing into the garage. I got out of a car in front of the driveway, ran over to my car, soon as he got in, the car out front was gone. I have the latest patch. R* is still glitchy w/ vehicles disappearing.