There's a fourth factor
Distance.
There could be, right now, several civilizations in other parts of this galaxy, and we may never know. Finite c, coupled with signal attenuation by the inverse square law, complicated with absorption by intervening dust and gas, sounds to me like a recipe for non-reception of evidence, regardless of wavelength... and it gets worse with increased angular separation.
Living near the central plane of the galactic disk is a bitch, ain't it?