Wow.....1 inch equates to about 500 gallons in your 20x40 foot pool. 2" loss in a few hours is a lot of water going somewhere. Very strange that you get NO loss when the pump is off but if that is really the case then something is opening up under pressure and is virtually closed when the pump is off. If the plumbing is PVC the only way I can imagine getting a leak that behaves like that would be a very long crack lengthwise in a pipe. If the pressure test was done right it certainly would show ANY crack. The leak would also have to be in the return side since if it was on the suction side you would suck air. When I pressure test my pipes I blow the water out and cap the returns. I then break the joint after the filter and hook a pressure test jig to the return pipe and pressurize to around 10 psi and it should hold for hours. The only reason I don't go higher than 10 psi is I don't want to blow the lid off my chlorinator.
I find it hard to believe there is no loss with the pump off. May be very small. You can try turning off the pump and letting the water settle and get still for a while then squirt some dye into the returns and see if it gets sucked into the return pipes. Try capping one and squirting dye in the other and visa-versa. Just for completeness squirt dye in the skimmer too. A big assumption on my part with the dye is the leak is below the pool water level.
Al