FWIW, we are also on a hill and have an 18' X 36' in-ground pool (30-35,000 gal.), 40 years old --- cement block walls, but with new vinyl liner. Our water level was dropping 2" overnight and called our pool guy. He discovered something had fallen in and punctured a dime-size hole in the liner at the deep end. Water was seeping through the hole, through cement bottom, through the gravel below that, and into the ground and away from the pool. Other than the level drop, there was no exterior indication of loss, nor could we see the hole from above. He dived down with a patch and cement and we were good to go. We have also had some leaking in a return line, but that was visible via a soggy spot on the lawn next to the pool.
@ passedpawn, we also had a crack in our skimmer box which was leaking *slightly*. Pools --- a never-ending source of "enjoyment".