Something you can top up yourself - any number of traders on here can supply.
Probably just a case of fluid going down with pad wear, but keep an eye on it just to make sure there are no leaks.
Might like to check back through the service docs to see when the fluid was last changed but if it is looking on the dark side would suggest a change is necessary, something I would do if I had any doubts as to its effectiveness. At my last service I had Hel lines and Motul Racing brake fluid added - certainly notice better inital 'bite' but yet to fully test under hard prolonged braking.