Hi, welcome here!

Like Al, I'm sorry to have missed this til now

If the water loss is noticeably less with the pump off, it indicates a return side leak.

Is the entire area between the pool deck and the pump house concrete, or is there some yard involved?

Areas covered by crete can be checked (with ~ 50 - 50 chance of success) by using geophones (or Al's trick of a nail in a stethoscope) when running pressure through the line with the other end plugged to narrow down which section of concrete has to be ripped out.

Please keep us informed as to your situation (I promise, I won't miss it again )