It can't siphon out of the filler neck, the filler neck is the highest point.
Gas sure can expand and then overflow through the vented cap. I've had it happen on several occasions with a full tank or mostly full but parked sideways on a hill. I learned not to park filler neck downhill or fill the tank very fill on a hot day unless I'd be driving right away to reduce the fuel level enough to not worry about it getting warm and burping.
Gas tanks always have an air space for expansion, or they are not supposed to be filled plumb full, leaving some % for expansion.
