If the ceiling vent in the bathroom is open when you're driving: it can pull air up and out of the bathroom... This creates a slight negative pressure in there, and that can draw the fumes up out of the tank.
If the air admittance valve is loose: it's just a natural way for the "ugliness in the tank"; to find it's escape route...
The fancy vent caps are cute... but certainly not a necessity.
(They just appealed to my sense of humor!
)