I'm pretty sure my bike has a vent tube, you can see it when you take off the seat.

 

if you have a cali bike, it will probably be different, but on a normal one, the tube on the back of the tank is a drain tube for the area around the gas cap. so when you spill gas or your beer in there, it drains out.

if you stick your fingers down in the gas tank and feel over to the left, you can feel the tube.

if you look at the inside of the gas cap, you can see the rubber that seals the tank, and that the hole for that tube is outside of the seal.

 

crazy, i was just about to post that same thing happening to my bike today, except on the way to class. I was stopped at a light and when i started out again the bike stalled. It really felt like it ran out of gas. At first i thought i might have stalled but I tried starting it a few times with no result. It wasn't very fun with all the traffic, but some guy that works at a bike shop got out his car to help. It was embarrasing when he came over haha cause it started right up.

I'll have to check my fuel tube tomorrow n see if something is up