When cold wipe down engine bay entirely especially expansion tank as Slick suggested and all hoses. Dry clean. Top off coolant with BMW coolant only (assuming you have not made mistake of using another type) 50/50 mix with distilled H2. After 48 hours park car out of sunlight (for easier viewing) and drop a 2'x 4' cardboard / paper under engine. Take a good look with a strong flash light or other light [even in day] for dried coolant. Look along seems of expansion tank, radiator, hoses, hose ends/clamps. I am pretty certain after you do this it will be less of a mystery.

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'14 X1 7000 miles

Hasta luego a

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