The simplest proved to be the problem.
Small line from the RO to the dispenser at the sink was the culprit.
Lots of clues that sent us in other directions. Plumber initially suspected the icemaker supply line. Never thought the ro line with a slow leak could have done this, but, sure enough.
Big result, beyond messing up the flooring, was that shutting off valves at the manifold trying to trace the source resulted in leaks starting there. So a replacement manifold is on the way.
