Common problem in a german X5-Forum as well. As far as I remember, some 20+ cars just in that forum have the same problems (excessive battery drain). People believe it has to do something with the Efficient Dynamics programming. Some people changed battery to a GEL Battery - seemed to work for some of them, but not for all unfortunately.
Me myself had a S-class in family that had to be plugged in literally every night to have the battery charged... until we found out it was absolutely necessary to LOCK the doors when leaving the car in the garage overnight. Otherwise several electronical devices will "test start" once in a while to enable a rapid start up when needed.

__________________
Since 09/2008:
BMW X5 3.0sd, spacegrey, black leather perforated, bamboo, nearly fully equipped
Car history:
05/2003 - 08/2008: BMW 325Ci fully equipped
1998 - 05/2003: Audi A4 2,8 quattro
1993 - 1998: BMW 318 Touring