Literally, turn your camera on yourself and stepdad and record the initial meeting of "hey, I'd really like to learn as much as possible about this.

I feel like they are taking advantage of my compassion and generosity, and essentially scamming me for lower rent while they make the extra $5000 income that I gave up by renting to them.Additionally, just since I met them - in the last 7 days - they have:  Disabled the security cameras on the property, asked my electrician to change light bulbs in their bathroom, moved furniture into MY AirBnB unit on the property w/o telling/asking me or my cleaning lady, unsuccessfully attempted to remove the bidet from the bathroom, and told my cleaning lady that she needs to raise her rates to me! 

You could also try security camera to see who is dumping.

It uses AR tech on your iPad to estimate distances by the gyroscope and camera in your device and is pretty much licensed as the engine for most of the other AR floorplan apps on the store. 

It is as well in a gated community and very secure with cameras at the main entrance...

Or invest in security cameras with an internet connection so you can constantly monitor everything happening.

A good team who can drive street to street and put eyes (camera and video) on the target 2.

I don't know if you've ever seen those old "caught on camera" shows but there was this one guy who use to secretly film "hobos" and one in particular that i remember is a guy who had a "jumpsuit" of hobo gear.

I keep a camera in my car and snap a picture of all signs that say "I Buy Homes" or Stop Foreclosure or whatever.

And, if you're concerned, using and alarm system, and or cameras would likely make you feel a bit more comfortable.