pool leak?

if a pool loses water and then stops when it gets past a return line ,isthe leak underground or at the liner? if my son-inlaw plugs that return line then the water level will stay above it. when running the filter he gets alot of a ir bubbles coming out of that return. any ideas?

Air Bubbles usually mean a leak in the Suction side....not the return.
The Return lines are generally the same height as the suction.

You can have someone pressure test the lines to be sure. It would help if he could close off one section of the plumbing for a few days to test.

Keep a bucket of water in the pool. That way you can see if the pool is loosing more water than the bucket due to evaporation.

From what you describe sounds like the leak is in the return line at the return jet level. Could be just behind the liner at the pipe penetration. Common place for a leak as the pipe is rigidly attached to the pool shell and if you get any frost heave or ground shifting the pipe can snap. Easy way to tell is to unplug the return and squirt some dye at the jet and see if it gets sucked in.
Al

thanks for the replies