Our pool definitely has a leak, starting about 8 weeks ago or so. Much larger water loss than normal. I have been letting it go to try and get a feel for when the leak is. IG pool, BTW. I did several bucket tests and am confident that I do have a leak.
Initially, I thought it was in one of the skimmer boxes (because we seemed to lose water quickly until the level reached the bottom of the skimmers). But, once the water was below the skimmers, I taped the fronts and filled them up - they have been holding water steady for at least a week - so no leak.
I did a quick dye test around the returns, stairs and fiber optic light ports, and checked the liner as much as possible (no obvious tears or holes, no soft spots) - so, no obvious leaks. Now the water level is just at the top of the returns. And, I notice that the return in the deep end is now blowing lots of bubbles. Turn off the solar and SWG to make sure they are the cause and they aren't. And, the water level seems to have stabilized. I also put a fitting in the return to make sure that the air for the bubbles wasn't being drawn in right at the return, and used some butyl tape around the fitting on the pool side to prevent any air leaks there - still bubbling vigorously! (I did check the filter and all to make sure that the air wasn't being pulled in on the suction side, no air in the pump basket, no air in the filter, no bubbles in the shallow end return).
I am planning on pressure testing both of the returns (have to disassemble quite a bit of plumbing, so I haven't done it yet). It seems a reasonable conclusion that there is a leak in the deep end return line that is currently acting like a venturi and pulling in air. The lines are all flex PVC and I'm relatively certain that it is one piece from the filter side return all the way to the wall return fitting. I just can't visualize how a pressure side leak could be introducing air. Anyone have any ideas?