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My guess is BMW decided to use a Model S as a dev/test/prototype/hack platform to accelerate product development. Perhaps they dropped one of their range extending motorcycle engines in it as a backup generator, like in the i3, to see how well it worked with a larger battery. Nothing to stop one manufacturer from buying another's car and doing some custom mods on it.
FYI - It may or may not be a BMW owned car. Just because the surround has the name of a BMW dealership on it does not mean that he car is BMW owned. Just like here in the US, one can have any license plate frame around a plate. I am not sure, but I think the M in the beginning of the number says that the car is registered in Munich though.
At Teslive last year, Goerge B told a group of us at the reception that a German manufacturer (he wouldn't say which one, but I figured it was BMW) had shipped them a car to convert to electric. A week later it was ready and Tesla shipped it back. George and others went to Germany to meet with the company execs about the conversion. When they saw the car the execs didn't understand that it had already been converted and assumed Tesla has returned the car with have done anything to it. They couldn't believe it had been converted in a week. If I have the story right, they then called in their engineers, who had been working on en electric conversion for a year or more, and reamed then a new one. So BMW engineers may be great with ICE technology but with electric it seems that they're out of their league.