To me, judging just from the pics available, the top of the sensor looks to have some residual rust, I presume from the flange it sits on, and it has dirt stuck to the grease, I highly doubt it's metal, but who knows. But looking at the top of the flange, the hole for the screw is in perfect condition, no rust. So I can only guess, that water somehow can enter through the knuckle and from underneath, not from the top of the sensor.