One problem related to coolant leaking has been reported for the 2012 BMW 135. The most recently reported issues are listed below.
When I had fewer than 50k miles, my thermostat broke suddenly while I was driving. I had no previous issues or warning lights. Since that repair, I have had 2 coolant leaks, and one related pipe crack. This week, I had air in my coolant system and had to have it drained. These all appear to be related.