I used to hear a screwdriver rattling around in the wing of my Cessna, but after several searches could not see it. One day, following some pleasing chandelles, I took the wingtip off, and sure enough, there it was. It belonged to "Hitch". He lost it, I still have it.
Years ago I brokered the sale of a friend's Piper Arrow. I took the owner to be to meet my boss at the time, who maintained the plane, perhaps to continue the maintenance in the same shop with the new owner. As the owner wandered to the other side of the plane to admire his new toy, my boss desperately whispered to me: "I'll pay you overtime, I need you to remove the wing tank, I left my flashlight behind it years ago, and have been promising myself to remove it, but have never taken the time to take the wing tank out (it is a big job). I can't afford to have the plane go to another shop and have it found there, my name is on the flashlight.". I pondered an overnighter doing this task. I had a look around the wing, including the landing gear bay. There was an inspection panel inside the bay, I removed it, and within easy reach, was the flashlight. I replaced the inspection panel. My boss was now having a pleasant chat with the owner to be at the other wing, and I walked over. "Oh, by the way Rick, I found your flashlight, here it is.". He had a very shocked and puzzled look on his face!
These are not the only tools I have found in airplanes, or cars back from the shop, for that matter....