As your brake pads wear, they get smaller. Thus more fluid remains in the caliper, and the level in master drops SLIGHTLY over years, probably. Brake fluid sensor is under the reservoir. Simple small plastic part, goes for about $5, takes less than 5 minutes to swap out.
BTW: they do go bad, just changed mine this year. You don't need to bleed or flush just to change the part, only if you feel the need.