I beg to differ...
Gas is a liquid and as it warms it WILL expand in volume that's physics.
Ever had a sealed gas can in the sun or even in a warm garage.
My garage will raise 20 deg between dat and night. Gas can expands during the day and the shrinks during the night.
I had a 56 Ford pickup. Gas tank in the cab.
If I filled the tank in the morning and then it sat through the day I would come out and have a cab full of gas fumes
The fuel in the gas station was cold from being in an underground tank then the truck cab would warm up and the fuel would expand enough to fill the filler neck and leak past the gasket. It would then dribble down the filler neck into the cab and down.
Turning them at key was always a thrill
For the OP....
A thoughts...
- Don't fill the tank till it clicks, purposely underfill the tank by a gallon or so and see if it happens again.
IMHO
If it did not leak when filled then the ONLY thing that can cause the fuel level to increase is expansion from temperature change.