I don't think anyone has figured it out so far. Here is another picture. It is less obvious in the photo but the piece of plastic is black, not the dark brown of the tensioner pads. Also it is too thin to have come from the pads. It also seems to be curved, as if it came from a piece of tubing.
At least no one has come up with a suggestion that it is a piece of a critical part, so I am going to class it as one of the universe's unanswerable questions, and not worry about it.