Hundreds of years ago in a former life, I was a young RAF Officer "holding" on an illustrious Vulcan squadron.
One December morning I was in the Ops room when in walked the Squadron JENGO (junior engineering officer) carrying a 3 legged milking stool. The stool had been found in the back of a Vulcan, in the tail-cone. It had obviously been there for a very long time. It was determined that when the tail cone had been rivetted on (either during construction or later at a deep inspection) the chap who had bucked the rivets had used it to sit on. When he worked his way forward he'd left the stool in the tail cone!
Tools do get left behind!
Very recently I lost a chum in a YAK52 when a screwdriver had become wedged in the aileron circuit.
Stik