Audi, VW, Fiat and others put some batteries in the trunk or under the seat. The batteries installed at the factory had a vent tube built in the top of the battery
. It was a plastic tube about 3/8 " in diameter that attached to the top of the battery case and exited below the floor. Many of these have been replaced with regular non vented batteries which does present some safety concerns but people do it to save a few dollars or because they are unaware of the difference. All lead acid batteries should be vented in some manner.