@Becca1564
As the vast majority of our guests arrive and leave by plane, I don't fear them returning to do something unsavoury. Most guests are really good and respectable anyway. Nonetheless, there's always the possibility they've given it to somebody else. And that's worth protecting against.
But we have an automated key code system that changes the key code on morning of departure. It automatically extracts the last 4 digits of the phone number used in the booking, and that becomes their key code from checkin time throughout their stay. Also keeps logs of when it's used and snaps a photo at the gate, so if there's ever any question about who and when they entered...
Also, we have various (trusted) service people who have their own codes, and know very well it's logged, so they know better than to give it to others.