I have a battery drain problem.Dealership and 3 mechanics can't find the source.

Does your vehicle have factory navigation? I've seen mysterious battery drainage problems that lead back to the navigation module in the luggage compartment. You can unplug it and see how it affects your battery issues. Good luck.

I had a similar problem and it was due to a faulty temperature regulator that sits behind the lower dash and is accessible from the passenger's footwell. This regulator controls some sort of fan which would continue to run an hour after turning off the engine thereby sucking the power out of the battery. Simple replacement by my mechanic that solved the problem. Apparently common issue in 2003 X5's due to bad sensor.