It’s a small plastic one like you would use to hang a poster or calendar. Sink drain
I can't imagine it doing any harm and it's not worth the effort to remove the trap to try and find it as it probably has already passed the trap and is in the sewer. If you like to fish, you could fasten a magnet to a piece of string and lower in into the drain and try to snag the tack. Just don't lose the magnet down there. You could also remove the trap but that can be messy and why take it apart if it's not leaking.