From browsing the different web sites, I gathered that the STi is made stealth, by changing the LO design. Instead of running the LO in the 10+ GHz range, like everything else (an RDD just has to tune into that known frequency to look for it), it runs the LO in the MHz range. Because of its low leakage, and tons other things operate in the MHz range, it is impractical to detect it, without picking up everything else as well.
The video below seems to suggest that the leakage frequency is 432 MHz. Which falls in the UHF band for HAM radio and TV. So there should be plenty of things emitting at and around this frequency.
YouTube - radar detector bel STI driver detection
Is my understanding correct?