Um I don't think you're correct there. The nub of the sensor gets worn through when the pads wear down enough to expose it. I mean, yes, the rotor does wear through the sensor, but only when the pad is sufficiently thin to allow this to happen. The sensor is there to monitor brake pad wear, not rotor wear. Your rotors could be paper thin and that alone will not cause the indicator light to go on.
In this case it sounds like it might be just a bad connection. Check the connectors to the sensors to see if one has worked loose or fallen off. Could also be a defective sensor, which you can determine by checking for continuity through the sensor with your multimeter. Also check and see if the section of wire between the sensors has been pinched or broken.
Checking for continuity (this one is good):
