F800GS overheating

This was going to be my last ride to Cornwall this year but sadly my reliable old BMW F800 let me down.
I had checked the oil and water as I always do just before a long trip and all was fine, I had no problems with it going to work and back. And when I pulled up at home after work and the bike was running fine. Loaded up the top box and got wrapped up in multiple layers and set off at about 1700hours and had only got about 10 miles when the fan came on and the temp had risen slightly above usual. Seeing as it was only about 4 deg ambient air temp I thought this a little strange so I pulled over and checked the reservoir and it was fine. As you need to strip the plastics off to access the radiator cap that was not going to happen at the roadside. So I let the bike cool down and set off slowly and turned around at the earliest opportunity and headed back home, I had to stop 1 more time before reaching home. And decided the this would have to be looked at another time as I needed to get to Cornwall tonight for Marcia's birthday. So the bike was locked in the garage and I set off to Cornwall via the van. Thankfully traffic was not to bad and I arrived at Marcia's about 2100Hours.
We had a great weekend celebrating Marcia's birthday drinking and eating with friends. The weather was cold and wet so probably not a bad move going by van. Now back home I have had a look at the bike and although the header tank was fine the radiator itself was low on water and had an air lock. I have absolutely no idea where the water has gone and why the header tank was still full. Now with a full radiator it seems to be fine. I will just have to keep an eye on the temperature gauge.