> You cannot passively detect another passive receiver.

In general, there isn't really such a thing as a passive receiver. The typical radar receiver design will downmix the incoming signal to some lower frequency (IF) using its 'local oscillator'. Some of the energy from this oscillator will leak out of the receiver and be detectable. This is only one of several possibilities for detecting receivers.

So no, not undetectable, but much harder than detecting an active transmitter.

This is almost always true, but there is at least one interesting exception: http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd...

I imagine for dedicated listeners, there might be others.

Are you joking ? How do you distinguish this "leaked oscillation" coming from radar-detector from all other pieces of electronics your car is stuffed up with ? Say, from your car stereo ?