[Overdrive]

I don't think I mentioned anything about any sensors. What I'm saying is that (apparently) the idea is that as the pads wear down, the fluid level in the system will lower, and once it reaches a certain low point, the brake light turns on on the dash as a means of telling you that your brake pads need to be replaced. Could also be as you said, for some reason or another the fluid level got low and needs topping up. So it's just something to check before topping up the fluid, because it could just be time to replace the pads before you top up the system.