AA993TT

I just got back from a DE at Beaverun in PA. I had a sleeping bag up front for the ride there and back and noticed a "stain" about the size of a basketball on the bag with a fuel odor. This has happened before. Last year I was giving a presentation and had a cardboard display stored up front which also developed this "stain". On another occasion I had a yellow Post It that looked like it was wet on 1/3 of it. Any idea of what's going on? I had my local shop check it out and they said everything checked out fine. Only advice they had was not to overfill the tank. In other words, once the pump stops, dont add anymore, which I never do anyways. Thanks for your help.