I spoke to a BMW 535D owner recently- he complained of a permanently flat battery -BMW's response was he did too many short trips, even went flat after driving to Spain though! you are correct about ecu shutting down in stages - I think 535D took 4 hrs to close down completely.
I have a 530D, this is my third one and I've never had a battery issue. But then again the car does get used a hell of a lot.
I think the solution maybe to find a decent auto electrician and get an opinion, seven days does seem one hell of a short time for a battery to go, we have a number of vehicles with active tracking and left without being used the battery will last several weeks. This does suggest a more deep rooted problem
