make sure the dealer is doing a key-off battery draw test. sounds like something in the car is daining the battery over night. Make sure your interior blower fan doesnt stay on after you shu the car off. this is 1 semi common problem area. Beyond that unless you have a meter that measures amperage and a strong electrical background and knowledge of BMW bus systems then unfortunately you are stuck at the mercy of a BMW dealer or a qualified BMW independent shop.
Best of luck,
DT

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DT