I finally found the leak that has been plaguing me and thought I'd offer up a tip that might help others. What finally homed me into the leak location was that the liner around that area was not tight against the wall. I knew the approximate height of the leak from where it seemed that the water level stopped falling this spring, so I swam around the pool with a mask on feeling for a tear/hole. As I did this, I noticed that one section of the liner was loose - it has some water behind it. I looked a little closer - and yay there was the tear! The leak height happened to be at the same height as my return line and bottom edge of the skimmer, so all this time I was suspecting that it was a leak in the pipes or at the interface of the liner to the return port or skimmer, but food coloring around those areas never showed anything. I didn't see this look-for-loose-liner advice anywhere, so thought I'd offer it up. Now I just need to patch it.
One question - I've read the patching advice here and it seems straightforward to apply, but do I need to worry about getting the water out from behind the liner or will that just go away on its own? I assume it will bleed out into the ground.
Thanks as always to this forum and community!