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My '14 M5 also did this when I first got it last year.
The solution was to replace the reservoir cap. It snaps on/off and has a part number 61667238068. This is #11 in the diagram attached.
The seal is pretty flimsy and I think the engine heat kills it after a while allowing a lot of evaporation. The part is very inexpensive and no tools are needed to swap it.
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