Jon wrote:

Has anyone encountered this spontaneous opening of a service valve before?

Are you sure it wasn't just slightly open when you left it and it's
taken several hours to leak enough water to fill the cup and spill over
(assuming it's vertical)? I've been helping a friend with his bathroom
and we've left several of these valves in place with no problems at all.

--
Scott

Where are we going and why am I in this handbasket?