Plasma doesn’t give the kind of radar reflections that have been seen on occasion (not always).
I wondered that too. The imaged (multiple spectrums and better resolution than any of us have seen,) craft were visible on radar and laser tracking systems.
Reports from military personnel stationed at nuclear weapon silos suggest that whatever these are they are able to interfere with the functioning of their silos. Maybe radiation from a plasma could do this but less likely than an intelligently controlled craft.
Come to think of it plasma is the driving force of up and coming stealth because it scatters radar.