Aha, yup! They are, indeed, the retention clips from the black buttons. Sometimes they can't take the stress of being permanently tensioned inside the cut-outs the buttons fit into and just break off :/
Honestly, if you haven't had any problems with your black buttons being loose just toss the clips away and carry on. If you do have any problems we will, of course, happily replace them. I'd just apply liberal amounts of glue, though ;)
We've been looking at dozens of button types and other things to find a solution, but right now it's a rare enough occurrence that it's not at the top of our loooooooooooooooooooooooooooooooooooooooooooooooong list of "stuff wot needs dun."
(You could also use them as tiny, tiny doorstops)