You're missing the point. There is no way a detector can tell if a radar signal is a cop or not, it just can't. Radar is just an electromagnetic wave on a certain frequency, it doesnt have a protocol encoded in it for communication that a radar detector can use to tell if its a cop or a door. All it can do is scan a range of frequencies that the FCC has said radar can work on, and if it detects a signal within that frequency range, it will warn you, and show you the strength of that signal, because that's radar.
A radar gun sends out a signal, and listens for the return of that signal bouncing off an object, then repeats this a fraction of a second later, compares the time it took for each signal to "echo" back and determines the speed of that object. There is nothing in that signal that is different than what automatic doors and security systems use.
Its like a wave in a pool. If you make a wave with your hand, or a boat makes a wave, there's no way to tell the difference, its just a wave (granted the wave would be larger with a boat, as a radar signal is stronger from cops due to the range they require, and is hte reason we can pick up police radar with greater range than doors and security systems).
If you have a radar detector that doesnt give you false alarms, then it is not as sensitive as the higher end detectors, and therefore is inferior. If I'm on the freeway, I want as much notice and sensitivity as possible.
