I had already paid a plumber the amount he quoted ($287), and then he sent me a $750 bill because he lost his camera in the drain! 

Hi, Other than locked doors , lights, video cameras, and security systems, there's not a lot you can do about wiring, or at least I have not seen anything.

I saw the video cameras at the entrances, but thought that he was just concerned or had nicer things, or both.

Get an alarm system.Also, consider installing dummy video cameras (unless you want to spring for an actual video surveillance system).  

Also keep your camera on your phone going entire time.

For instance they have remote cameras you can put inside of the place when they left that upload feeds online.

If they are stupid to show up hoping to collect the deposit dont forget to take their pic with your phone. just remove the flash and put the camera on mute.Hopefully you can catch these low life!!!

Josh found the CAMERA.

Some use security cameras to monitor their properties from another state.

Getting a copy of their driver's license might also be a good idea (just take a pic with digital camera).Going through that helps to weed out some of the bad characters ... but not all unfortunately.