When I take my gas cap off for a fillup there is no hissing sound. Does anyone elses ride do this? Most cars you hear the release when you take the cap off. Just noticed it so I don't know if it's something new.
I read that there is a valve between the gas tank and charcoal cannister that opens when the pressure gets so high. So at a certain pressure, the vapors will go to the cannister and await a PCV event that is suitable for vapor disposal. Pressures below that certain level, would be maintained in the gas tank, unless you remove your gas cap and get hissed at. Maybe your cap leaks?