When i bought my secondhand 2011 F25 X3 3l diesel some five/six years ago it had about 130k kms on the odo. When i drove home from Stellenbosch to Hermanus with it the day i bought it, the coolant light came on. Stopped and had a look but could see coolant in the bottle. Now, about a year ago my son had some issues with the same car which was passed on to him. It was discovered that the reservoir bottle was leaking on the bottom and had with all the years of slow leaks gunked up around the fitting at the bottom of the reservoir. My point is, check the bottom of the reservoir on your car for maybe a similar problem??