Your locale may or may not be favorable towards the return of old parts. Some jurisdictions require you to hammer out an agreement that the old part be returned prior to the contract being agreed to(verbal or written). Others make the mech be obligated to return the item if you request(although don't wait too long). But, I would at least make the attempt to demand return the caliper. Because they might have used it for their own gain.
I mean, I recall a mechanic who took even the small "Shift Lock Release Button Cover" on my mom's Matrix, a tiny circular thing, and he was being paid by insurance to do body work.
https://parts.lakelandtoyota.com/p/...ton--Shift-Lock-Right/3942378/3356302030.html
Seriously, taking a freaking $15 part.
While I don't have personal experience, it seems like good practice to mark parts with an identifier, such as permanent marker, paint, or engraving(even a crude one with a cold chisel) prior to handing custody over to the mechanic.