No, seriously, it's passive. In fact I believe they are using cell phone towers as signal sources.

Stealth technology depends on reflecting incoming signals in a different direction, and absorbing the rest. However if you have signals coming from many directions, more of those signals may be reflected to the detector.

In fact, if you also have many detectors and can synchronize them, you can detect plenty of return. So the stealths have perfectly flat bottoms, meaning if they maintain a level flight path, the radar won't pick them up until directly overhead because the signal will bounce to the ground somewhere else. Now if you have detectors over a large area, you can detect the bounced signal and figure out where it came from. With enough "benign" transmitters and detectors, you can detect returns without overt radar transmissions.