Water seeping out of grout in exterior corner of tiled shower curb
This is not an answer, per se, but so far has solved the issue.  

I called the original tile installer to take a look.  He pulled the tile piece that is to the left of the leaking grout corner.  We inspected the liner and anything else we could see.  The leak must have been very slow because there was minimal damage inside--no wood rot.

The exact cause of the leak could not be determined, but the tile guy sealed the liner edge behind the curb corner's vertical and horizontal tile pieces with some kind of rubberized sealant (not silicone) to make a water dam.  Then used extra tile and grout mix we had to make it look new again.  So far it is holding up.  Then again, the leak is very slow.  Hopefully, I'm not back on here in a few weeks or months with a different answer!