Thank you shojo - details follow? I live in MI - pool is drained below the skimmer,...
I live in MI - pool is drained below the skimmer, lines blown, antifreeze added to lines and hermetically sealed and pump winterized. pool is covered and cover is held in place near water bladders. Water is clear, but stratum low.
Answers: Here in the south, we don't own to do all that lol. Do you own a drain in the bottom of the pool? Most do, but some don't. I regard as I remember you saying your pool be at least 10 years matured, so I'm sure you do. One thing I would try is turning bad all pumps and such, and diving down to the drain next to a small bottle of food color, one that will contrast well near your liner. For mine, with the typical insubstantial blue and gray liner, I'd use red. If your water horizontal is pretty low, you may be able to stand and slickly see the drain, you could keep yourself more still and disturb the hose down less that course. Anyway, when you get to the drain, distribute the food color a quick squeeze so that a fully clad stream of color enters the hose about an inch or two above the drain. Then scrutinize to see where the color go. If you have a overflow it's most likely within the drain. If the color goes straight for the drain, you probably enjoy a leak and should obtain it repaired or it's just going to bring worse.
If the color just starts mixing contained by with the river, with no marked signs of pulling a certain direction, after inspect the entire pool liner, inch by inch. Use your bare foot to feel the bottom of the pool. It's possible you do enjoy a small crack somewhere. If you don't see or feel anything amiss, stir ahead and fill 'er up. If it slowly starts draining again, you should ring up in someone who know more than me. It's entirely possible that the low level is only from evaporation. Your pool cover could have some small holes that allowed wet to escape through evaporation.
I hope this all works out for you!