Help With Cooling System

[Paulpt]

I wonder if any of you can help. A few days ago, my check engine light came off. Refilled the tank, made sure the gas cap was fitted correctly and waited a few days to see if it will reset. It did not. Checked oil, was OK, checked coolant and was down. Refilled, drove a couple of days, rechecked coolant and it was down again. Started checking for leaks and saw none, other than the plastic plug right next to the refilling cap was craked in the middle. With the help of a reverse bit took the part that was still inside the thread and replaced. I did replace also the cap as the gasket was paper thin. However is seems that there is still a leak coming from up there, I checked the hoses and other parts it seems it is all OK and I can't see where the leak is coming from. The check engine light is still on ( the car does not overheat) but I am in NY and the outside temp is very cold, I guess that is the cause. Has anyone experienced something similiar? help please and I suggest all to check that plastic cap (I think it is the air release cap) and the radiator (or coolant reservoir) cap. These parts are cheap $15 for the radiator cap, 3 for the plastic plug. Thanks in advance for any help.

2001 X5 3.0

Paul