Thanks all. I actually emailed Leonora, but got a call back from her colleague at BMW North America and she opened up a case for me. I also got in touch with a different dealer BMW Gallery in Norwood who offered to at least look at the vehicle to see if they can determine if it is a warranty part causing the power drain. As they kindly reminded me, if it does turn out to be the subwoofer wire, then I'd be responsible for it's removal outside of the warranty...and that was always understood, but they explained in a perfectly civil manner, rather than the threatening and accusatory tone Herb Chambers employed.

The previous owner took the sub out before they traded it in. The subwire is underneath the door panels or carpet and wired/tapped in somewhere to the main power. I'm still not certain that this is the actual problem, but I have no issue with taking it completely out of the car. My only concern is if it is soldered in rather than simple wire taps. That goes beyond my capabilities.