You found your fault!
The airhead gauge on a hydronvane - should leak off after the airend has stopped rotating - you should also hear it leak down - thru the air intake filter.
If the tank to airhead one way valve has failed or stuck - the airhead gauge will stay reading tank pressure (till the contents of the tank have slowly leak thru the airhead).
I am sure you would have heard this thou?
Hmmmm. Ive heard nothing. Holds pressure for days when left. Are you saying, that if the valve was stuck open, the tank would empty through the air end?
Im now doubting myself. Im there tommorow morning to check, but basically, if i have 100psi in the tank, the air end gauge, the one in the middle of the black plastic cover should show zero?