the smell is the moisture trap inside the a/c system...when you start the car the odor should be the strongest. the problem is...you are using the re-circulation a/c all the time. 5 minutes before you park the car...turn the a/c to fresh air and this will eliminate keeping moisture inside the a/c duct system after the car is turned off. the odor will remain for a couple weeks but if you follow my suggestions the smell will be gone for good.

my prior M3 had the same problem...this is what bmw suggests and i works perfect to prevent the odor.