These battery drain issue seem to crop up occasionally on the forum; but I can't recall anyone ever finding a definitive solution/culprit. Rather than scrolling down the list of threads, try the search utility using a few key words like "radio" and "battery drain." The consensus seems to be that some module (in your case, perhaps it's the radio head) isn't going to 'sleep' when the vehicle is locked.
By design, the radio head is supplied with power for a period of time after the engine is shut off, or until a door is opened. Sounds like something in that path isn't getting set properly... Either way, my memory tells me that the radio/clock display should go out immediately after the engine is shut off. Over time, this could be the source of the battery drain; I had a similar problem develop in the radio head of my '97 GMC Jimmy, and the battery would be dead after only a few days of sitting idle in that state. You may need to employ the same solution I did for my Jimmy (replace the radio head)... In the short term, you may need to try pulling the fuse on the circuit that supplies power to the radio head when the ignition is off (which is, again, what I did with my Jimmy until I could obtain a replacement radio head).
X3 w/6-speed & Sport Package:
A sports sedan disguised as an SUV...
Last edited by timfitz63; 12-07-2012 at 10:31 AM.