Well... my Trouble Free Pool has been indeed trouble free for this whole season. Now it's only trouble free from a chemistry standpoint. Lately I've been losing about 1/8" (3 mm) of water per day.
So far, I haven't done any investigation other than a 12 hour bucket test where the pool lost close to 1/8" more than the bucket.
I know I have a small leak in the PVC plumbing between the filter and heater (darn screw in adapter fittings...) but this isn't large enough to explain this level of loss. I lose maybe 10 gallons per day at the most through this barely present leak.
My vinyl liner is in good shape but has several patches from previous owners. I'll be investigating the patched areas and skimmer/drain face plate areas first using food coloring test. I thought maybe even before that I'd cap the return lines and skimmer lines at the pool to see what is lost overnight.
I'm likely in for a long investigation unless I get lucky and pick the right thing to test first.