I'd look at security cameras, especially since you own the house next door and this is the only place that seems to be affected. 

I have the following at my Vrbo:Commercial camera system.

Most plumbing companies now have cameras that can see inside the sewer line and see what is wrong with it.

I learned that when a home is vacant and people break in chance are they will continue to break in have a camera system installed

I learned that when a home is vacant and people break in chance are they will continue to break in have a camera system installed

I drop shipping containers with camera's wifi and remote locks to keep materials in and have delivered to.

I travel a lot and can see when the rents are dropped off through a lightbulb camera so there is proof of any activity near the mail slot.

Cameras - Security cameras at all entrances, in the hallways, etc. 

So the tenant complained at a recent quarterly inspection that the laundry did not drain well (washer drains into utility tub, then to sewer line.) plumber who came used "camera" down roof vent pipe and said there was a crack in cast iron pipe somewhere; charged for the visit and advised to call specialized pipe service to video to find crack.

Security cameras, double locks, install blinds so people can't see inside, leave on lights, be active in the property so it looks like someone lives there, have something to make noises like a tv, leave a car in the driveway