Spy cams definitely have their usefulness. For awhile we were having security issues at the house we owned in Mexico, so we set up a “trail cam.” We got a beautiful photo of the back of one sneaky person once — then they found the camera (which was concealed in a palm tree) and took the batteries out of it. End of that experiment!
But spying on your guests? That’s just not kosher. Renters pay a fair price to have privacy in the unit, just as they would in a hotel room. I cover all of this in Host for the Most, but basically: don’t have things in the house that are so valuable people will be tempted to take them, and get a security deposit in case of damages. If you can’t live with some level of risk — this is not the business for you.