The sensor is in the brake fluid reservoir. It's a float with a little magnetic reed-switch. You've checked your fluid already, so try unplugging the wires at the reservoir cap. If it quits doing it, then your float switch is flaky.
The 'sensors' for worn-out brake pads are just little metal tabs on the brake pads. They scrape on the rotor making a screetching noise.
BTW, when you push the caliper pistons in to make room for your new pads, don't let the reservoir overflow.