After replacing a bathtub, I noticed water in the ditch under the P trap, about 15" below the floor level (15" below the top of the slab). If I run water anywhere in the house for about 15 minutes, the water in the ditch goes away (seeps back into the earth). When there is no water running in the house, the water seeps back in to the same level consistently. I have run this experiment more than a dozen times, so it is definitely related to the drain system. Several plumbers have not been able to explain it. A video down the main drain line showed nothing except there are two bellies (low spots). Definitely a drain pipe (not a pressurized pipe) issue. Any thoughts how to locate and fix? Thanks!

A leak in a water line that only drips when a valve is shut?

Good idea, but the water still returns even if the water supply to the house is shut off. Also a professional leak detection service determined that there are no leakes in the pressurized water system.