Trane central air inside unit leaking water through square hole.

My Trane heat pump inside unit leaks water during the most hot and humid part of summer ever since it (and a new outside unit) was installed in 2009. The PVC drain pipe is NOT clogged, the plastic pan is not leaking, but every year the installing contractor says the water is coming from ice on the coils that - when it melts -- overwhelms the capacity of the drain pipe to carry it away.

Today I saw water coming from under the door where the inside unit is. So I turned the unit off and removed the panels so I could see this supposed ice on the coils. No ice, but I saw water dripping out of a square hole just to the left of the drain pipe connection. And in this video, I poured some water in the drip pan, and it all came out of the square hole and went under the unit --- It CAN NOT go out the pipe because this hole is lower than the pipe outlet. This hole must be for some kind of sensor that detects the water in the pan about to spill over --- it's mentioned as an optional feature -- but otherwise it must be plugged or the water will always come out of this hole. Am I correct or is there some other explanation?

Видео Trane central air inside unit leaking water through square hole. канала Frank Provasek