Has happened to my old man. He woke 1 morning to go to work. Goes outside, no car in the driveway. Goes back in to get keys for mums car and comes back out, sees a guy driving his car back into driveway. Guy gets out, looks at the old man, says thank you and runs off into the distance. We worked out this guy was regularly taking the car and bringing it back. Many arguments where had about not putting fuel in the tank. Turns out, this bloke was using it all. Lol.