I've had a lot of bad batteries the last few years. Some even loose their charge being disconnected. Others being connected even the smallest load like what the clock memory draws is enough to make a difference even if the battery holds a charge when it's disconnected.
If you have the ignition switch light make sure that goes off. That stopped timing out on a friends car. Unless there's something aftermarket the only real stuff on an M to cause drains is problems with the radio, ignition module, or a light that's not going out. Or the alternator thing that was mentioned altho I've never personally seen that problem.
I thought about the ignition light but either it's burned out or disabled due to AHB. Factory radio so minimal draw there. As you mention, not much on M bodies. I'm going to test again with alternator disconnected. I did leave negative cable disconnected yesterday after testing, just to see if that changes anything. I probably won't have a chance to get back to testing it again for a week or so anyway.
