Hole in water cap - Why?

I just overhauled and put together a 1996 2.4 with automatic transmission. I had (poor) help removing the engine and did a solo putting it all back together. I started pouring coolant in and suddenly had a leak. There is a 1/4 inch factory bored hole in the rear water outlet cap at the back of the head where the radiator hose connects. This hole is just immediately above the right / back hold down bolt hole. Why does this hole exist? is there something that belongs there? Is it supposed to have a plug that fell out? Thanks. PS. The car runs great otherwise.

I've just solved my own problem. I got a chance to look at another car and found a return line from this cap back to the plastic reservoir. Now I only have to find the steel line that goes there. As I said poor help taking it apart.

Thanks anyway.