Centric rings are plastic or aluminum so yep thats them be sure those find their way on to the car as well!
Insist that you can watch and supervise otherwise they may end up damaging a 1500 dollar brake system Im sure they wont mind lol Nothing they will be doing could endanger you at all so i cant see why they wouldn't let you stand guard. The shop would be at fault if something was to happen tho and you would get a new brake system...