The brake fluid reservoir has the warning sensor in, the light doubles up as the handbrake warning light. It doesn’t have to go much below the full marker to trigger the light. Sometimes just cornering can trigger in when below the full mark. Also the brake reservoir doubles as the clutch fluid reservoir.
Just to add, the caliper could be seizing and or slide pins, need a clean up and lube.
Maintenance is the key, many posts about seizing brakes. A regular service (annual strip/clean/lube) not a glance over of pads and discs by whoever once a year is good enough.