My inground 18,000 gallon pool lost 9 - 12,000 gallons of it's water within three weeks of closing, this year and last. I barely had to add any water (miniscule amounts a couple, few times for evaporation). The Pool and equipment did not leak each summer, but each winter, it does. I have two main drains but, it doesn't completely drain out. The neighbors yard is lower than mine and he has a drainage system in his yard so, I don't see any wet areas? I am stumped...any ideas?