I had the same problem with my polecat about 2 months ago- problem was a very slight leak in the bung ( cap)of my tank- how did i find it- well, I insert my tank backwards( bung towards the rear) and upon removing the cockpit moulding then noticed the fibreglass moulding of the fuselage at the TE of the wing moist with fuel.............did a pressure test and yes that was the problem.
All is ok now. Suspect though that this was made easier by the rearward tank, trevor henderson, NZ