A recent post to the always-entertaining HotelChatter blog discussed a not-so-new scam thieves are trying (apparently with some success) to foist onto hotel guests. In the scam, the thief calls a hotel, asks for room 520 (or whatever). Once connected, the scammer pretends he or she is from the front desk, saying there is a problem with the guest's credit card info. The would-be thief asks the guest to read back the card number and the secret 3- or 4-digit code on the back.
I don't see this scam working in most hotels, because as part of their training, hotel telephone operators (or whoever answers the phone at a property) are told not to put calls through to rooms without asking for the name of the guest. It's a simple way to avoid this kind of scam or even something worse, such as an attack on a woman guest.
While that may be SOP at most hotels, this would be a good time to reinforce that rule for anyone at your property who could allow a guest to fall victim to such a ruse.
While it may ultimately be the guests' responsibility to not succumb to such a trick, you as an innkeeper have a moral, if not legal, obligation to prevent this kind of crime.