The low coolant indication showed up on our 2011 35 around 7K miles. Added about a pint of BMW coolant (you could probably just use distilled water, given the small amount) and, at 25K miles, haven't seen any other indications. Would seem to support the "expansion" theory. A number of other threads have discussed this topic. Very few reports of real leaks, although they do happen.
Dave @7500 feet on the plains in Colorado
2011 X3 35i - Blue Water Metallic