That's probably an evap system leak. There are two components that can go bad, the square box connected to the gas tank, and another one on the engine. I've had both go bad, I *think* the one on the tank is more typical. The one on the front can be a pain to get to if I remember right. Do some googling I think you can even test these things once they come off by blowing air in them. (at least the front). The one in back is fairly easy to change, and you do not need to drop the tank or anything ridiculous like the manual says...
I think this thing can even be caused by a bad gas cap, anyway there's info here to review.