Well...it is runoured (and I stress rumoured...) that the F-117 in Serbia was tracked partly through the phone network, before being killed visually when in range. The active radar was supposedly left "off" to avoid anti-radiation missiles, and only turned on briefly for confirmation
Think about it - all you need to know is the standard electromagnetic pattern in an area. Anything passing through it will case perturbation. All you need to do is have suitable detection equipment to measure the changes - its not much of a conceptual jump from monitoring magnetic anomalies produced by submarines...
If you push the technology far enough it should be possible to use other transmission systems as well - for instance the national grid power lines, which kick out a heck of an EMF field. Or more interestingly perhaps - its now becoming clear that fluorescent street lamps can be used for generating radio signals. If they were A:LL set to radiate in a known pattern, detecting low level anomalies shouldn't be too difficult
OK I'm talking hypothesis and conjecture here - and a lot of research cash. But it should be do-able
But to answer the question - if the defenders can avoid detection they don't need the passive radar to guide the missile. They just need to wait until the attacking aircraft is known to be in the kill zone - and then use other precise targeting.
Think WWII - the Chain Home / Chain Low stations didn't guide missiles. They simply guided other aircraft who then made the targeting decision.