You can't. Don't bother. Once they noticed it was missing, they undoubtedly notified their bank, who would have cancelled the card and issued them a new one -- so the one you found isn't good any more.
If you really want to do something with it, bring it back to the issuing bank, or just throw it in a mailbox and let the post office return it to the bank it came from.
i just hand em into the bank.