Leaking antifreeze

I just had the same issue with my 318. After parking the car I would have coolant under the car. I assumed it was a expansion plug as well and checked al them and they were dry. It was the header bolt on drivers side in the back. I'm not sure if which one it was so I just put rtv on both of them and no more leaks. Just like Mullimax suggested. Be ready to lose coolant to the ground when you pull out the bolts.