The pool was losing 1.5 to 2 inches a day while the pump was on. I finally had eliminated the side drain and the two skimmers. Even though I had done a water column test on the main return line and it did not show any loss, I didn’t see what else it could be. So I started digging it up:

Got down to the pipes and connections and it was bone dry.
So that is the good. None of the elbows were leaking.
The bad was that I had gone against the evidence and dug up the main return connections for nothing.
I just went back into the pump room to turn the pump again when it suddenly hit me that I had heard a cracking noise several weeks ago when turning the multi-port valve from filter to backwash or vice-versa.

SOOO, I turned the pump back on and went back down to the waste pipe, and sure enough, water was dribbling out. The valve was leaking water from the main return into the waste pipe.
Took it apart and sure enough, the gasket was all kerfluggalled (the one on the left is the bad one)
and the gasket channel was broken.

The even better good news is that Feeley’s had a replacement gasket which solved the leak. I have to order a new valve and replace it at some later point.