After 30 min the right tank level went from full to 1/4 in the space of 5 minutes. Left was indicating 1/4. A look out the right showed a little fuel staining from the fuel drain valve.
This comment is a little telling to me...and begs me to ask a question.
Did you confirm that the fuel drain(s) were correctly seated after checking the fuel/water etc..? (not trying to teach you to suck eggs etc, just wanting a clearer picture of what you did) As this method of leakage is a trap for the unwary.
Secondly, on preflight inspection did you notice any fuel stains running away from the drain areas...and potentialy also on the leading edge of the flap, and or flap rod?
Otherwise if you'd done all of this...I need a better idea of where else the leak may have originated.
BTW I'ver never trusted a Cessna fuel guage
