Most modern car batteries are of the sealed type. This means that the acid in it cannot escape and that they have a gas recombination system for the gases that are produced during charging and discharging. Vented batteries, however, were used in an era where gas recombination batteries were expensive and charging control systems in cars were rudimentary, to todays standards. Vented batteries are not maintenance free, because the oxygen that is vented cannot be used to recombine into water anymore. That's basic chemistry. Even when a battery is of the "sealed" type, gas can still escape through safety valves in the cells. Batteries should therefore always be charged in a well ventilated area, which the engine bays is. However when a battery is placed in a different, non ventilated area in the car, a designer may prescribe a vented battery connected to a tube to the outside. Without knowing the battery make and type, there's not much to say. IMHO such battery does not apply to an Avensis.