I completed the job today.
.... and went downstairs to put tools away and encountered a fresh pool of water right in the middle of the floor !
I looked at it, said the appropriate words, and then noticed that a cardboard box on a shelf six feet up was damp.
Flashlight time in the rafters.
It was right underneath the bathtub.
But the drain pipes were all dry.
I enlisted my sister [here for a visit.]
She went up and filled the tub, then opened the drain.
That part was OK, but as soon as she opened the tap there was water dripping.
It's the &$^&%%&^%* faucet, up in the wall !
Looks like I know what my next project will be.