I went to bed with my pool looking like this:
Woke up to it looking like this:
Yep, I lost over 4 feet of water during the night! My pump was set to run for 2 hours after sunset so this drainage occurred in just 2 hours! I discovered this morning that the drain plug for the filter canister had broken off and the pump sent 18,000+ gallons of water out into my backyard. The good news is, I was here to prevent any further loss when the pump tried to restart this morning!
My guess is I over-torqued the drain plug when I recently cleaned the filters (Aug 30) and the plug finally snapped in half at the base of the threads. The moral of the story is: DO NOT over tighten a drain plug!