i was about to flush out the radiator on my 1998 306 1.8 xr the other day when i went under the car and saw what looked like and empty space where a bolt (plug) should be. I could see the faint outline of the head of a bolt left on the metal.

It was located around near the sump on the opposite side to the sump plug (i.e the radiator side) and was roughly inline with the middle of the car.

Anyone have any idea what this is for? i thought it could have been the drain plug for the coolant but im not sure. Also if it was this then shouldnt the coolant be always running out barring a blockage? I also found a fair bit of dirt in the top of the radiator header tank.

Thanks for the help