My 900ss has a Lil bubble in the tank just down from the cap.. it all makes sense now!The only thing that might be worth looking at is the paint on the top surface of the filler neck, where the cap will seal against.
I would try & rub it down to the bare metal, just on the flat mating face though. If petrol gets under the paint surface it will travel along the metal, under the paint & the paint will start to bubble/lift.