Probably the aircon condensate drain tube, no reason for any other tube to be inside the cabin because the heater plenum drains are IIRC outside the cabin and down the firewall. It should drain aircon condensate out of heater assembly - that is the pool of water that you normally see left behind when you drive your car away from parking spot in hot weather with aircon on, if you are parked on a slope it is more like a little river of water running out from under the car.