I just checked mine and I'm seeing the same thing, never noticed it before. I haven't had mine long enough to really notice it before but mine definetly has what you have. I would like to know what it means as well!
Specific to Deimos's questions above, my bike is not running hot based on my temp guage and my coolant is in good shape as well.
2003 Yamaha R6 stock
2002 Yamaha YZ426R Dirt Bike stock