Piece of a leg brace, perhaps? Like the kind worn for a major knee injury?
Yep, either that, or BBC is a leading indicator of the template. CNN is saying both that it isn't a medical issue (stumbled), and that it is (she should release medical records or have a presser with her doctor asserting no issues).
If the incident is merely discomfort and stumbling, it isn't a medical issue at all.
-- In the last video (#3) it is clear that a metal object falls from the right pant leg of Mrs. Clinton. --
I have a copy of what I think is the best/original, and I do see something fall, just as the move starts. Interesting.