I had a coolant smell for as long as I could remember on my Defender. Took it in the other week and turns out the smell was caused by a tiny pinhole leak in the auxiliary radiator, which was so tiny that I never got any low coolant warnings. Turns out thy found literally hundreds of tiny rocks in my aux radiator, and the leak was caused by one of said rocks hitting the radiator. Presumably these rocks were kicked up by a truck on the highway.