Satellite orbital elements used to be public knowledge
I think they, or some other gov't department, published orbital elements. They shouldn't be surprised if most satellite positions are approximately known by some astronomers, and can be extrapolated decades into the future. A hostile country might have radar that can track them directly, and have missiles that can destroy the satellite. But I don't know how an amateur would get his hands on radar, or what he would gain from it. I wouldn't call him an amateur anymore if he did.