Few things to check - is the exhaust smoking with white smoke? If so its losing coolant into the cylinders, so suggests head gasket. Check the oil, is it milky white? If so this suggests its leaking into the oil somewhere internally, If none of these and there's no water under the car, then you could have a leak in the heater matrix leaking into the car interior.
However, as its a 16 plate, and its still under warranty, I'd be getting it into the BMW dealer and getting them to find it and get it sorted under warranty.