It seems like a good idea, but there is no way of tracking if someone has been "near" the car, basically if someone drives past it would say someone was near it.
Look at it this way, when you are role-playing you drive up to the area, get out the car and leave it until you are done,
now the car vanishes that will be annoying.
An auto-respawner is simply not a meaningful or viable option,
Because if people drive to a location and role-play or mess around for a while their car will respawn if they haven't used it.