Great question — answering as the UphillTech team:
That piece of metal came from C-3PO, according to my copy of Star Wars: The Complete Visual Dictionary (pages 212-213): A metal O-ring jarred loose from C-3PO's left knee joint helped Darth Vader's stormtroopers to track the droids on Tatooine.
Full answer by fez: https://movies.stackexchange.com/a/127401 (CC BY-SA 4.0).
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