Location of small black plastic part shaped like right triangle

Hi All,

A small black plastic part, shaped like a right triangle, fell out while I was tearing down my son's iPhone 4s for repair. I searched the interwebs and couldn't find anything like this particular part.

As shown in the attached image, one side of the part has a circular-shaped impression/depression in it, and the other side is flat. The circular depression is not a hole and does not go all the way through the piece. Any idea of where this piece came from?

Thanks in advance!