I thought these things burned up during re-entry?
Mostly, yes, but sometimes they split up and pieces that are denser can make it to the ground...............
If it’s NASA junk, they intentionally minimized the impact of something off the ISS surviving. I’d not have given the piece to them, but would have called a lawyer and evidence specialist first.
Really dense pieces make it to the ground. Also extremely not-dense pieces make it to the ground.
Dense: small presented area, too much material; doesn't completely burn up on the way down.
Not-dense: slows very quickly due to relatively large presented area and little mass, doesn't get hot enough to burn up.
F=mA.