-Thanks, Phisherman. I believe it was, indeed caused by an
intermittent leak. However, fortunately the dog that had it has moved
to a new residence now ;-)
I was worried the urine might somehow decay the wood over time. But
thanks for your advise to the contrary.

-I think that might work, cybercat. But I suppose after the stain
removed, that area will no longer color match the rest of the floor.