[Twoforty]

I appear to have the same problem with my newly acquired M240i. (15000km).
Had this same problem with the B48 in my 330i. (30000km).
The dealer performed pressure and no leak could be found....
HOWEVER, at my insistence, they went further searching for a mystery slow leak and actually found it!
They removed the under cover below the engine and discovered coolant leak from the water pump. See image.
This seems to be one of the most common problems with these new engines. B48 and B58 are similar design.
The pump was replaced and problem solved.
Now I got to go through the same thing again with the 240, which is still under warranty.