The newer systems would have a vent line coming off the the tank that snakes up the hollow area of the cab on the passenger side then back down and out the floor (this acts like a trap in a sink drain to keep fuel from coming through the line)...then snakes up to the engine bay and be connected to to a charcoal canister. Theses tank systems would have a non-vented cap. All the swelling and contraction of the gas during temp changes would suck and blow through the canister (not the cap) which is supposed to take the smell out of the vapors. Here's a thread with good diagrams...
http://67-72chevytrucks.com/vboard/s...d.php?t=662158
Edit: I'm betting your garage gets hot during the day and much cooler at night causing alot of the 'sucking and blowing" of gas fumes from the tank.