the fuel gauge would show as full until the trapped air escapes, which realistically would happen when driving, especially over bumps and round corners. then you'd notice the fuel gauge dropping faster than it should.
the air will rise to the top without you needing to take the cap off. if you take the cap off and put it back on, how will that make any difference? air still rises with the cap on. try it with a bottle of coke if you doubt it.
lol. was assuming that becasue their is an airlock in the system that the tank was pressurizes (as the label says behind the cap) so the air wouldnt neccessarily move....