After having this problem, I have disconnected the fuel line at the gascolator and plugged it.. and I still get a fuel drop in the header tank.. Yes the fuel should not go below the tank return line to the wing under normal conditions but if the plane is moved or the wind is acting on the plane the fuel will move around and drain back to the wing until such time the level stabilizes below the outlet.. You can also test the shut off valve by disconnecting the fuel line and observing any flow into a vessel. Bob
