Most likely a crack in the rubber Y-pipe from the fill hole to the tank. Mine has the crack as well, and when you fill up there is a small puddle of gas on the ground. You can probably get the hose from FCPGroton or somewhere similar, and I think on the installation you don't need to drop the tank completely, but maybe a little to be able to reach all the clamps and have room to remove and replace. At least that's all I can remember from reading up on how to change it in preparation for me to do it eventually.

-Steven