If after reading this you don't understand what I'm trying to say, please go out to your car and pop the fuel door flap and it should be pretty obvious.
On most cars, you screw down a gas cap until it "click, click, clicks" and then you know it is sealed. Not so on the GT. The GT has a plastic seal that slides SIDEWAYS as you put the fuel nozzle into it. The sideways sliding piece has a "ramp" built into it that tries to convert your nozzle insertion motion (no lewd comments, HHGT) into a 90 degree sliding motion. Anyway, it is not uncommon for this seal to get stuck in an open or partially open position. It will be hard to notice - so you have to make sure you look each time. Make sure this slide goes back to its fully closed position. On HHGT's Shell experience, I'd bet that these may have been cases where the slide didn't close all the way.
As I said, if you don't follow this, it may behoove you to go outside and give your car a little fingering to see what I'm talking about. There's a risk that your significant other could catch you in this act. This is why new owners shouldn't try this yet. It's one thing to get caught with you finger in the GT and it's another to get caught doing this why you are aroused at the same time. New owners need to wait until they build some resistance!