I currently own a 2013 BMW 320d, generally a great car. Now at 32k miles I have noticed the coolant has fallen below the min level (2k miles after its last service).

This also happened at 14k miles, which i topped up and had it checked at its 15k service. The system had no obvious leaks when checked by my BMW dealer.

With the car nearing 3 years old i am concerned it has a leak somewhere that is both very slow and hard to detect that will present itself once it is out of warranty and cause further problems. Are there any known issues with these cars?