Back on really old ZF trannies I removed from Audis, there would be an air vent at the top of the transmission case. It would have a small metal cover crimped on to it that you could move to the touch. Of course, this was also in the days of dipstick based check and fill systems and three speeds.
This sounds like the same thing, other than they have put an extension hose onto it for some reason. Maybe it is either to get it a bit further away from the heat that would be back there w/ the exhaust just above, and/or to get the air outlet effectively higher up if the car were ever even momentarily submerged up to the level of the transmission tail. The cover is presumably to minimize dust ingress, or water or other stuff getting sprayed int it somehow from above.