As others say your best protection is recording a valid credit/ debit card number and expiry date. When a guest offers to post the key back I swap the barrel and key between this and another room until I receive the key back. I do not have a master key but one key goes to the guest marked with room number,a second key marked with room number and spare stays in reception and a third key on housekeeping ring. In 11 plus years only 2 keys were never accounted for. Just a note, recently it took Aussie Post 9 days to get a key back to me it was posted in an overnight bag just 57 kms. away from here.