Even if you're not in one of those nanny states, nothing good can come from this situation.

Get security cameras and a security system. - It is just money.

Decent security cameras with digital recordings and good lighting do wonders.

I would recommend taking a camera and taking pics of the docs and you'll find it's a tremendous time saver.Good luck to both of you!

To all you experienced landlords, short of installing cameras, how do you guys catch illegal subletting?  

Assigned/Carport parking Pet rent Storage units View/location premiums Automated package storage/Amazon lockers Wifi doorbells Security cameras Add washer and dryers Stainless steel appliances Update light fixtures Upgrade bathroom (cabinet, sink, shower) Upgrade kitchen (cabinets, sink, counter-tops) Build a fence around ground floor patio  Install vending machines RUBS program (Ratio Utility Billing System) Charge for garbage pick-up Add a daycare GymCost Segregation (had to add that 😁)What would you add?

The more the tenants can see, the more likely they are to inquire.Make a Walk Through Video No, you don’t need a professional video camera.

Arlo camera with a speaker is not expensive, may be gate the community too.

Warning: This tactic may be less effective in the era of high-definition exterior video cameras.

You don't know where - further isolation testing and a camera are required to determine the specific location of the leak.