What you are experiencing is called parasitic draw. This is actually somewhat common.
If you have any kind of aftermarket equipment installed - which you do - this is the first suspect in any parasitic draw situation. I would suggest removing the aftermarket equipment first.
After the non OEM equipment, the blower motor speed controller and electric engine compartment cooling fan are the most common causes. The airbag computer, battery sensor an idrive/nav computer are also somewhat common.
Beyond these items though, it could be ANY component on the car.
In order to determine the cause of this (assuming it is not the aftermarket equipment), you will need to monitor amperage draw and then pull fuses one by one until the draw stops or drops below about 50ma. Whichever fuse stops the draw is the problem circuit and you can identify the issue from there.
BMW dealers also have special computer equipment and tests they can use to help identify the cause of this, so this is a second option, besides the above test.
Please let me know if you have further questions. Please do not forget to click a positive rating as this is the only way I receive credit for my answer, thank you. Best wishes with your vehicle