> you give one of these to your AirBNB guests or a building maintenance worker or something, and if you get your keys back, you can be reasonably confident that they didn't make any copies.
This seems to be a fairly useless reassurance. If you trust your AirBnB guest so little that you think they might reasonably make copies and distribute your key to unsavory people, then just the single key floating around out there is enough to compromise your security.
The risk is that someone will rent the place and spot an easy opportunity to copy the key and come back when other guests are staying at your Airbnb. Having a known number of official keys out there with a guest means you can keep tabs on those key and retrieve them for "full security". But having an unauthorized copy puts the owner's and every future guest's security at risk until the lock is changed because you cannot retrieve it or even know if it exists.
This seems to be a fairly useless reassurance. If you trust your AirBnB guest so little that you think they might reasonably make copies and distribute your key to unsavory people, then just the single key floating around out there is enough to compromise your security.