My spa has an intake in the floor, a return on the wall, four bubbler jets on the wall, and....the curious part. On the bench part that's formed into the spa itself, where you sit, on a hole in the wall of it by where your calf would rest. It's plumbed into something but I'm not sure where it goes. What is this for? I've asked a couple of "pool guys" but they just guessed. Hopefully someone here knows.