Quote from BMW document:

SITUATION: The MK3 Navigation computer powers down in approximately 60 seconds after the ignition has been turned OFF (red LED on the computer goes OFF). If the power supply to the MK3 unit is interrupted (by removing/disconnecting the computer from the vehicle, or by disconnecting the vehicle's battery) in the time period of 55 to 57 seconds after the ignition has been turned OFF, the power-down procedure ("erase and save data in Non Volatile RAM") can be corrupted. This will result in a total failure of the MK3 computer (red LED off, CD eject function inoperative, board monitor screen permanently blank, and no sound from the audio system). In such a case, the navigation computer has to be replaced.

CORRECTION: On vehicles with MK3 navigation system, prior to disconnecting the battery or removing (disconnecting) the Nav computer, make sure the MK3 Nav computer is in the power-down mode (red LED on the unit is OFF).

End of quote

Not a great chance that power off happened exactly at the wrong moment. I think it is something to be careful about in the future but nothing to fret about what may have happened in the past. And then again “total failure” as far as the dealer is concerned does not mean it can not be recovered by more imaginative individuals.

Anyway your system does not show the right symptoms for a dead unit. I would not lose faith.