I think you have stumbled across the temperature sensor inlet port. The hose goes (via an internally mounted temperature sensor) back to the suction side of the heater motor. A small volume of air is thus drawn by the temperature sensor from the cabin of the vehicle, allowing the climate control computer to make intelligent decisions regarding heating and air conditioning controls.
'82 300D/4.3L V6
'77 Jag XJ6 (Has the same temperature sensor setup)