My father, in WW2, was in the signal corps, and in England in those days all receivers were licensed. Nazi spies had radios that were licensed, as they did not want their radios detected by their IF(Intermediate Frequency) radiation. Superheterodyne radios tuned to various frequencies by means of a low power variable frequency oscillator that was mixed with of off air frequency to make the IF output of the sum and difference which were selected by tuned IF transformers and amplifiers. They would listen for these signals and drive to maximize them to see where they were tuned. Usually the spies also had a legal radio operating in the hopes that their common IF signals would be lost in the general hubub of radio noise.

My Dad listened for the rejected signal - which was usually in the clear, although faint, with vans with sensitive regenerative receivers in them.

Once a suspicious address was found, that was handed over to a special squad who set a watch on that house, followed people and listed for transmissions. These transmissions were usually brief and once a spy was found he was analyzed in detail by groups of people who would follow him/her in a special way that passed the tracking off to another as they walked off to avoid the subject getting to know he was followed by seeing the same guy behind him all the time.