Why would an owner of a car like that leave the door unlocked and keys still in it? Maybe he was talking on his phone as he parked, and just kept on talking till he got into the house, completely forgetting those keys.
Why would 3 teens be trolling for unlocked cars?
Not an uncommon tactic for somebody who has, or is about to fall behind on payments. If you're going to lose the car anyways, better to have the insurance company pay off the loan with a theft claim than to have a default and repo on your credit history.