Probably leaking at the fuel vent line which is located in front of the shock tower on the clutch side, mine does this as well. There is a check valve on the top of the gas tank but these tanks seem to pressure up quite a bit in warmish weather - next time you have ridden for about 30 minutes or so undo the gas cap - you will be surprised by the amount of pressure.
I moved my vent line so that gas isn't coming out near anything hot, and I don't fill the tank very full either. My dealer doesn't know what to do about it??