I recently shut off the whole-house water supply when replacing 2nd floor bathroom sink faucet cartridges because I couldn't get the sink supply line shut off all the way for the hot side to close all the way. I opened the other bathroom sink taps to verify water was truly off and started working.
After maybe an hour, maybe a bit less, water started coming from the open taps of the second sink (which were open), and dripping from the showerhead (which was not). No water came from the faucet on which I was working - though I also had the under sink shutoffs shut.
Why did this happen? Did I just not have the whole-house valve shut all the way? If so, why the delay? Was water just slowly making its way to the 2nd floor? Is there something else going on here that I'm overlooking, like, should have also turned off the supply just to the hot water heater (though the whole-house I thought would trump that)?
All ended up fine, but I'm interested in any ideas why this would have happened.