Before topping it up (which is perfectly ok btw), check your brake pads. You haven't lost any fluid, the level has dropped as the pads wear, and a low level should be seen as an indicator that they have worn. Were you to fill it right up as it currently is, when you replace the pads and push the pistons back into the callipers the reservoir will overflow as there'll be too much fluid in the system. Make sense? :-?