A radar cannot function without some source of electromagnetic radiation. Passive radar[1] requires non-cooperative sources of such radiation. If they are det

A radar cannot function without some source of electromagnetic radiation. Passive radar[1] requires non-cooperative sources of such radiation. If they are detecting the plane by some means other than EM, then it isn't a radar. Now, their system may be using EM blocking as part of its tracking. The principle here is that a low-observable aircraft creates a "hole" in the background because it absorbs or scatters EM radiation away from the receiver. This method works best when the background is very noisy (i.e., there are lots of transmitters operating in the area). In that case the reflectors (and anything else that increases the radar cross section of the aircraft) would actually hurt the performance. [1] https://en.wikipedia.org/wiki/Passive_radar https://en.wikipedia.org/wiki/Passive_radar