Wednesday, I noticed the water level was lower than it should have been. I got the hose out (I'd put them both in the garage to protect them from the 20 degree temps) and filled it.
The water level continued to fall. Thursday morning it was down a bit more:
And by Sunday more than half the water was gone:
I called our pool guy and he came over and declared the liner dead. We couldn't find a hole but the cost to get a diver (!!) here to fix and attempt to repair it was formidable, especially since there's no guarantee a patch would work. The liner was not new - but we have replaced it once in the 11 1/2 years we've lived here. With age, it's gotten brittle, and the cold weather probably caused its early demise. I'll share pictures of the new improved pool in a few weeks!