I don't recall any fuel line or tank vents being routed under the seat, so I suspect the fuel must come from somewhere else. Fluids can easily run along parts of the bike (frame, lines) and end up collecting quite far from the actual source. Clean everything well and try to trace down where that fuel comes from. First thing I would suspect is that you overfilled the gas tank... is it possible? Have you just refueled, before leaving the bike on the side stand? Also, do you have the evap canister in place? If not, did you remove it? I've seen very poor jobs done by people removing the canister and not properly re-routing the tank vents...