I woke up this morning to our fresh water pump happily pumping away, but nobody was using water!
A quick check found that the hose going from the sink cabinet shut-off valve to the port side and then through the cabinet floor (at least on my boat), then to the washing machine had a hole.
Where it went through the cabinet floor it had rubbed against the floor enough to put a hole in it. You can easily check this by pulling a bit of the hose upward and feeling along the length.
Repair was simple, I simply cut the hose in the deck locker in front of the stove, and used a repair hose barb and ran new hose to the shutoff valve. I also took a piece of 3/4" diameter hose, split it, and put it around the 1/2" hose to protect it from chafe.
It would have been much worse if we had left the boat for the day without shutting off the water pump. Another good reason to shut off the water pump!