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Fuel valve. Your fuel valve is located at the underside of the wing where it joins the fuselage underneath an access plate. Worth a look to see if you are having some leaking there.
Right now my fuel valve is not providing positive cutoff of fuel flow. A small trickle happens in off position. This tells me the ball in the valve is not sealing all the way. (a project for the near future). Likewise, if the balls in your fuel valve are not providing positive seal for the ports to aux tank and main tank, they end up being in communication. So fuel can trickle or flow from one tank to the other; they are in communication at the valve.
So why would fuel move from main tank to aux tank? Possibly a pressure differential. Specifically the main tank getting pressurized. Possible fuel vent plugging. It's getting warmer now, fuel heating up and fuel vapor pressure going up. Check fuel vent for main tank.
I had a visit today from a fellow curious about my skymaster while I was working on it. He went on to tell a story of about two episodes of engine stumble and power loss in his plane. Long story short he had fuel vent plugging from dirt dauber nest. He cleared it by using weedeater line and measured to be sure weedeater line reached all the way to his tank. When he saw mud on the end, he lightly pressurized his tank blowing out nest fragments.
Not too long ago I cleaned my scupper drains which were plugged by dirt daubers allowing water to get into tank. Shortly after one scupper had water pooled in it again. I probed the drain and found a fresh insect larvae blocking it. Didn't take long for a bug to find it again. They like those vent and drain pipes.
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