The Plus 8, a Supersport with a trad body, does exactly the same. It seems to relate to the ambient air temperature. In the summer it runs at the mid to high 90s, but once the cooler weather arrives it is at 100 to 105.
I suspect the cause is a combination of factors, including as Alaistair said, use of the A/C, but as the pattern is predictable I've never bothered to find out more. If I feel it is a bit high I turn the A/C on and it quickly drops back to 98-100.
The header tank is at the back, just in front of the heater box, where it is on most if not all trads and even a slight weep would be obvious.
