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Guess a start would be to check the battery cables/ground straps of a 11 year old vehicle.
With a new fully charged battery the codes would be cleared and the reappearance of any subsequent codes looked for.
A new code could shed some light on the source of the problem.
Normally a check of the alternator output would be done at some point, but given the new battery was dead with the car just sitting for a few hours the alternator output is not yet called into question.
Then check for power drain with the engine off and after the car's electronics have had time to enter their low power mode.
A power drain on the order of (WAG) at/over 100mA is too high but of course the factory service/repair manual would provide a more precise number. It is probably less than 100mA but with the battery dead in just a few hours you aren't looking for a 110mA drain but one on the order of amps.
At any rate the cause of too high a power drain needs to be identified. This can be done by removing fuses and then installing the fuses one at a time until the power drain spikes.
Or by removing a fuse at a time -- starting with lights/accessories circuits -- then advancing on to the fuses in the more complex -- engine/drive train -- circuits.
Once you have identified the circuit responsible for the power drain then the excact cause/location for this power drain needs to be found.
The above is not a job for a DIYer although there have been cases when the DIYer was successful.
Now an experienced BMW tech might know of a shortcut to identify the bad circuit he might even know where to look as your car would not be the first one to manifest this behavior.
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