All radio receivers put out a signal, via the first IF. Perhaps they used that to track him. The WWII Germans used the first IF of radio receivers to track people listening to illegal(by German laws)radio broadcasts from the BBC. Remember you can be tracked via your cell phone even when it is turned off, as long as you have the battery in it. You have to remove the battery if you don't wish to be tracked.
From my experience with cellphones, I have always been skeptical of this claim, tending to treat it as an urban legend. It implies, of course, that when you turn your phone "off" it's not really turning OFF.
Now, there are certainly appliances with "offs" that are not totally OFF, such as remote controlled TV sets, etc. Another example is my Kindle, which when "off" still runs down the battery using its wireless links to get updates on my digital subscriptions.
But I have not seen any evidence that any cellphone, smart or otherwise, works this way. Admittedly, my personal experience is scarce; I haven't turned any of my cellphones of the last 20 years "off" for a long enough time to see if the battery drains at greater then its self-discharge rate, indicating surreptitious activity.
Anyone have links to definitive info on this? Myrddin? Bueller?
The receiver contains a local oscillator (LO) whose job it is to translate the incoming RF channel to a fixed intermediate frequency (IF) using a device called a mixer. The mixer combines the incoming RF channel with the LO so that the mixer's output is at the intermediate frequency; this is fed to a series of gain stages all fixed-tuned to the IF.
To tune the radio to a given channel, you just vary the LO so that it funnels the right RF channel into the IF section.
The rationale for the superhet architecture is that fixed-tuned gain stages are much, much easier to design than variable-tuned ones.
The most likely radiated energy from a receiver is the LO, which reveals only that the receiver is operating; and, assuming you pick it up and also assume that the receiver uses a commonly selected IF, you can deduce what channel is being received.
The next most likely radiated energy would be the IF signal itself, which does carry the information being received.
However these are both weak signals, effectively undetectable more than a few feet or tens of feet from the receiver in a cellphone.
Considering also how many cellphones would be operating (most of them in standby), it would be like searching for a needle in a haystack.
It is true that there used to be TV rating services that would drive a truck with a sensitive LO receiver and a side-firing directional antenna. It would sweep past the houses and could record which channel each home was tuned to.
By they way, cal, I suspect my pedantic explanation is old news to you; I just wanted to get it on this thread by way of general explanation for the less geeky. If, that is, any of them can stand to read it.
The German Funksicherheitsbedienung was very effective at tracking down covert transmitters. Their motto was Funksendung is Verrat, "Transmission is treason". They tried to impress that on the German Army, with their Enigma machines, but with little success.