What would cause a gas leak in the top of the tank only when the tank is full?

I was thinking fill tube. I put 21.5 gallons in. The most I've put in since owning it. It starting leaking at the gas station so I flew home. It was running from the top of the tank, down the sides and dripping from the bottom. It dripped maybe for about 10-15 minutes and stopped. I only lost 2 cups of fuel. Drove it to work the following morning and nothing. So far I've driven a total of 75 miles and haven't smelled gas or seen any leak. I think the o clamp where the fill tube meets the tank is has usted and fallen off. That's what I think happened.