An update. Since my last post I've run the pump/filter from the skimmer and from the main drain for 12 hours or more with NO discernably different drop from the bucket test. Loss since my last post is about 1/4 inch, easily accounted for by evaporation. The dogs seem to have drunk from the test bucket; it shows about an inch drop.

Talking to a dealer about a new liner he mentioned that a bit of trash could stop up a liner leak. Sounds reasonable. That could explain the sudden 3 - 4 inch drop, and its "miraculous" cure.

Someone on this or another forum mentioned that plumbing leaks accounted for only about 10% of all leaks. Since my plumbing seems to be all underneath concrete, with no indications of the slabs sinking, perhaps my situation falls into the 90% of leaks, the liner.

Any thoughts from The Collective Pool Brain???