That's wild lol. Had a similar thing with a bag of old buttons once. Just got the shop vac and sucked them all up before putting down the new pad. Homeowner was just as confused as yours.
The shop vac move is honestly genius, I mean why bother picking them all up by hand. It's always the weirdest little things that get left behind under old flooring. I found a whole collection of bottle caps once, like someone was saving them for no reason. Homeowners never have any clue how that stuff got there either.
After @paige_martin mentioned buttons, I'm all in on the shop vac method. It's a total game changer for cleanup. These finds are always bizarre, like someone's secret stash. Homeowners play dumb, but we know they hid that stuff. Seriously, why waste effort when a vac does it in seconds? Just suck it up and move on.