I had the 'high discharge' warning for several times now in the past weeks, so I drove to my local BMW dealer and had an 'energie-diagnosis' done.
Results showed;

1) too many short drives (out of 209 drives in 35 days, 148 shorter than 5 kilometers)
2) a battery that was only charged to 44% (40-49% in the last 5 days)
3) 41827 hours of battery usage (seems a lot..??) range of charge was most of the time in the 60-80% range as it should be). Normally the battery is charged to 80%, rest is done by braking-energy (efficient dynamics)

The technicians told me that there was nothing wrong with alternator and or leaking currents, nor was there a problem with the (3 year old) battery. Their advise was to change my (2007 OEM lead/acid standard BMW-Varta (white housing)) battery to an AGM Varta (black housing) together with a software update (different charging protocol).

They also told me another interesting thing; the first range of AGM batteries (2008 series?) had a capacity (?) problem and almost all of them have been replaced or will be replaced in the near future (??)

I guess at my next service I will change my battery, but for now I will get my Ctek charger, as the guys told me that my driving-pattern is absolutely not sufficient for charging my battery enough...
In conclusion: BMW has only recently recognized that the capacity of their OEM batteries is not sufficient for the kind of heavy usage that the average X5 has nowadays......