I have an inground steel wall, vinyl lined pool 16' x 32' straight back kidney shape. I live in London, Ontario. Closed the pool in the fall, lowered water to just below return lines, blown and plugged. Now Draining water off of lock in winter cover and noticing water level to be about 6-8" below return lines. What happened to the water?? Last fall the pb came and sucked water out from behind my liner that was around edges of the deepend but says they couldn't find any leaks and it might have been ground water that got behind the liner. However at this time noticing that the water level has dropped over the winter I'm thinking that there is a leak somewhere. The pool cover did get quite full over the winter and water level under the cover rose significantly due to the surface water on the cover. Trying to determine the cause of the water level differential?? Would like to address the issue before filling the pool back up when opening. Any ideas or help with this matter would be great. How to find leaks, fix them or determine if there is one or not???