I had the same issue several months back. I had orginally drove down my street to a friends. I was there maybe a few hours came out and the car woudn't unlock so I had to use the key fob. Just like you said the clock and a few other things had resetted on the iDrive and something about a battery discharge was displayed on the iDrive, but once the clock was set it went away, It did it 3 times that day before I went to the dealer, all they did was unhook the battery and retightend everything up. (Like I didn't think about that the first time it did it).
Total just that day it did it about 5-6 times. 2-3 more times after going to the dealership. All this was on a Saturday. The dealership said if it comes back to make an appointment on that Monday, but the next day it was gone and I have never seen it since and that was back around Sept-Oct time frame.