I do not know for sure but I think this may be a drain for the heater/air
conditioner case. If so, it would drain the water condensing off of the cold
AC evaporator coil and also any rain water that made it in there thru the
fresh air intake. If the radiator coolant leaks out of there, you have a
leak in the heater core. In some cases, a leak where the hoses connect to
the heater core can cause water to seep into the heater case and come out
the drain. Many cars do have a drain hose connected to lead the drain water
down under the car.

hope this helps a little.

Don Young