When you gave your son an opportunity to have a drive he might have liked it so much he's got the spare keys and taking it out for a blast when you are all tucked up in bed? Wouldn't be the first time that has happened.......
Trust me - he values his knee caps too highly! Tell you what though, I never thought of the possibility of them being nicked! I'm sure they'll turn up when I'm looking for something else! I hope!
and that something else will be your car......