Let's theorize for a moment:

Suppose you turn the engine off and the check valve allows fuel to seep slowly back into the tank. While the supply line is still pressurized, the fuel might pass backwards through the check valve fairly fast, but as the line pressure drops, that rate of backflow will diminish. Finally the line pressure drops to zero, and gravity is the only thing incouraging a backflow. But if the pressure regulator and/or the injectors allow air to pass backwards into the supply line, (and that may very well happen??), then given enough time, like over night, the supply line could become quite void of fuel and full of air.

Now, here you come bright and early in the morning. Turn the key on (pump runs) and ahhh, instantaneous pressure measured at the guage. BUT - are you measuring the pressure of fuel against the guage, or are you measuring air pressure which is being pumped up by the restart of a fuel flow back at the pump? If it's the latter, might it not take a short time to restore fuel to the rail? Just some food for thought.....

I've often thought of installing some sort of pressure guage also, or at least a means of "quickly" connecting one if I suspected a problem. I don't like the idea of running a line into the passenger compartment though. A quick-connected one could run out from under the rear edge of the hood and be taped to the windshield for "monitoring while motoring".

I'm just curious - How fast does the line pressure drop after you shut the engine off?