Have any of you considered the fact that the fuel tank is sitting directly on top of a very hot engine that will rapidly heat the fuel in the tank? To prevent serious issues with overpressure, all fuel tanks have relief valves in the cap that route the excess vapors to one of two places. California bikes have a small carbon cannister, similar to all automobiles. Forty-nine state bikes have a rubber hose that just routes the fumes out into the atmosphere.
The second area that emits fumes is that nice open air cleaner. Fuel in the intake ports after shutdown also vaporizes, and the open air cleaner just makes it easier for those fumes to escape. It's something I've noticed over the years when I've changed out the stock airbox for a SE Pro type air cleaner.
Jerry