As far as I have been able to see from the trunk, the tank has no leaks. It also holds a fair amount of pressure because the cap always pops off with a burst of air when I unscrew it, so that is why I thought that it wasn't the tank itself that had a leak. But is it possible that the leak is small enough to still hold pressure while allowing gasoline itself to escape? If so, how would I go about detecting the leak other than a mere visual inspection? If not, what are the other possibilities?