Sometimes, when using a socket to tighten or loosen a fastener; the socket comes detached from the handle, and remains attached to the fastener. I once purchased a '72 Capri with a bad engine; and found, under the valve cover, a socket still stuck to the tappet. I wonder how many miles that socket was stuck there, going up and down. I suspect that it may have had a negative effect on the whole valve train; and perhaps led to the engine problems.
That's my story and I'm sticking to it!
Also - in the crevices of my Dad's AH 100 (under the hood, between engine compartment and fender) - probably 15-20 different bolts, washers, and other miscellaneous hardware - some dropped in by my Dad over the past 30 years; and some pre-dating him entirely.