I have seen "thugs" checking for the keys while I was gassing up the tank in my other car in a questionable part of town. (It was a BIG old Caddy Fleetwood Brougham). I was at the rear of the car pumping gas and they walk up and start chatting me up about "would you sell it?" and "how much do you want for it?". They can just about climb right in and be gone before you can do anything about it. Maybe harmless but it can, and DOES happen.
NEVER LEAVE THE KEYS IN THERE AGAIN!!! Your kids might even end up "going for a ride" if they are in the car when this happens!!!!
Be safe, not sorry.
-Mark.