IF I overfill the tank and leave it on the side stand in the hot sun, I get a puddle under the bike. Last time this happened, I was still at the gas station. Filled the bike, parked near the store entrance and went inside to pay. When I came out there was some gas under the bike. The guy filling the tanks at the station said "your float is sticking" I smiled. Floats are in carburetors. I think he knew that and was joking.
With the bike on the side stand, the vent is closed. There is a weight that hangs down and is only open when the bike is level. I think the gas pushed out under the cap and drains out the water drain in the cap area. Look at a parts picture to see what the heck I am talking about. I think it sort of like a rollover valve in a car.
There are 2 vents one below the cap sealing area with the weight and one above the cap sealing area to drain away water (or excess gas).
David