You may not have a leak. If your plumbing is set up a certain way, you may have just siphoned out the water due to a selector valve that did not seat properly. I teated my neighbor's filter with a cleaner last summer. I left the valve in the backwash/waste position with the pump off with the intention of rinsing everything the next day. The waste line down hill acted as a siphon and several thousand gallons were pulled out overnight. I have also lost water during the season because the valve did not seat correctly after backwash. The system was filtering but also dumping water with the pump running.

That could have been a possibility because we do have a waste line that runs downhill during the pool season. However I checked and it was disconnected from the filter when the pool was winterized. I talked to the pool closing company today and they don't think there is anything we can do until spring. I just hope the walls don't cave in due to ground movement from freeze/thaw cycles.

There really isn�t much you can do now. You either have a leak in the main drain line (if you have one) or in the liner probably just above that level (or it balanced out there with your ground�s water table) - which you pretty much stated.

Hopefully, you will keep the six inches in your pool all winter to prevent the pool from "floating" which is when a pool wants to pop-up out of the ground because of outside ground pressure. When your water in there does thaw, you will want to monitor the water level and see to it that it doesn�t get any lower.