Bad gas tank relief valve?

Bad gas tank relief valve?
So my Oldsmobile (99 Alero) is acting up again. It's supposed to sit in the garage for the summer, but it wreaks of gas. There are no spots on the floor and no visible leaks anywhere. The smell seems to be originating from an open line that comes from above the gas tank (can't see if there is a valve there or anything). The line is grouped with the fuel lines and is made of the same hose material. I've heard from one person that some cars have a relief line in the gas tank that feeds back into the engine. My line is pretty short and I don't see any other open lines, so I don't think this is the case as there is nothing to connect this line to that I can find.

I see that some other cars actually have a mechanical valve-type mechanism. I would assume that a modern car would have a gas cap that would fulfill the need for this valve. Anyways, I'm guessing there could also be a valve above the gas tank that I just can't see/get at without dropping the tank. Maybe it's stuck open?

I'm a bit lost on this one and haven't been able to find much on exactly what goes with this hose that is making the smell. I really don't want to drop the gas tank if I don't have to as it's mounts are very rusty.

  giant016