Red or yellow light? If red, check if the parking brake is fully releasing. If not, the cable is stretched or one of the rear ones is rusted in the partially-applied position. Unplug the level sensor in the master cylinder. If the light no longer comes on, the float may have drowned. The master cylinder should not be full if the front pads aren't like new. Mechanics won't top it off during other routine service because when new front brake pads are installed the pistons must be pushed back into the calipers and the fluid behind them needs room to go back into the reservoir.