Shift detent ball.
Each synchronizer hub has three of these balls (and springs) between the hub and sleeve. It's not uncommon to have one or two pop out, and get away with the remaining ones. If shifting isn't horrible (and maybe more importantly, it's not slipping out of gear on its own), it's something you can forget about.
https://www.ft86club.com/forums/showthread.php?t=139737