When i bought my truck, i found a bunch of snap on tools tucked away never to be found again in the engine compartment. And i found 12 dollars worth of cash in the ash tray, can you say car wash.....):h
Lower alternator bolt most likely.
Maybe the upper AC pump bolt??
The motor mount bolts have a tapered end (Threaded end) to allow for them to go in easier and they dont use a washer but have a flanged head (washer head)
Likley during an alternator change at a shop the bolt went MIA and the tech just grabbed another.