Maybe keep a camera in the vacant units?

I'll have a fun part two thread where the other tenant stole moving out on camera and knew that was happening!

When I ran the camera the reason became obvious: all of the soil underneath this part of the building had washed away.

Buy some security cameras now.

There's a chance he may have dug into walls looking for spy cameras or hidden babies.At least he's communicating with you.

I provide internet on site (mainly for the security cameras).

We even had one bored tenant install a ring camera to check when the washing machine was done so they could put it in the dryer or check if someone else had finished.

I’ve had two plumbers run the cameras and they said that if the ground was dry in December when I inspected, it may not have been spotted.

I will always have a plumber with me and have him run a camera down pipes for any possible deals that I'm interested in. and have plumber and foundation around same time so you dont pay twice for digging.

If that still doesn't clear your mind, then you can go with a PM, or you can wait outside while they look and film them entering & leaving the house as proof you were outside, or get cameras outside to independently film everything that's going on so you can have proof you were outside, etc.