It seems the battery drain is quite common in BMWs.I read an article in BMW magazine of the M3 longtermer and has the same,the battery drains after a two day stop.One of the thoughts about this is the keyless go feature.Having the key in the pocket or nearby(for example your house) the car goes into standby mode as it detects your key.This requires energy and the car stays in standby mode for quite a while.If the battery goes into safe mode to secure the power for ignition you could loose all your settings in idrive.
That is at least what i read in this article.