Here is my input as well.
I have 2007 X5 and I had Battery discharge issue. I was reading these posts here and decided to definitely bypass BMW dealership. I am in Ontario, Canada and there is a guy here who is well know as Car Electronic Expert (I put capital E for expert because I think he deserves it). Anyway, he gave me two possibilities:
1. FOB key - if it's close to your car, car will never go to sleep mode - that wasn't my problem as my car is about 10-15 meters away from my key fob.
2. Battery - he asked me how old is that car, when I said 5 years, he said "not official rule but BMWs especially X5s need battery replacement every 4 years", if you go to dealership they would tell you "noo, it's not true, battery is good". He suggested to get new battery but battery from Series 7 as series 7 has more computers (chips) and will be perfect on my X5. It cost me about $300 for OEM battery and about $90 for labour and he said "if you have the same problem, come back I will give you back $390 immediately" - I still didn't go back to pick up the money even though it was 5 months ago which means no battery discharge problem since then. I hope it'll stay that way :-)