There are 2 possible sources for the beeping. Either a tight parking brake or the sensor on the master cylinder.

I discovered if I disconnected the connector on the master cylinder, the brake light went off.

Do this test to validate it is the sensor.

On mine, the fluid had drained below the sensor, but the light never turned on. I replaced the master cylinder and with a full reservoir, I still get the beeping.

I've been told a few panic stops could fix it. It has not.

I need to find some time to return the new master cylinder and replace it since it appears to be a bad sensor.