It sounds to me like the end that you stick into the pad is no longer insulated and is making electrical contact with semi-metallic pad and thereby to the disk. The sensor system sees the current and thinks the disk is contacting the sensor. It looked to me like my sensors had insulation around the part that goes into the pad, as well as what looked like hot melt adhesive on the end. Maybe your sensor was somehow defective. Since they are cheap, I believe I would try another. When you pull the sensor out you may leave junk in the hole in the pad and may need to drill it out so the new one can go in undamaged.

I bought my car from my neighbor. I listened to the rear brakes squeal every morning as she backed down the hill. When I bought it, I changed the rear pads to the Mercedes pad and they have not squealed since.