A bit of ancient history for you, taken from Hansard in January 1952, this article explains how the TV detection devices of the day worked.

Receiving Sets (Detection)HC Deb 30 January 1952 vol 495 c34W 34W
§ 70. Mr. Donnelly asked the Assistant Postmaster-General if he will circulate in the OFFICIAL REPORT a brief statement of the scientific method employed by the vans used by the Post Office for detecting wireless receiving sets.

§ Mr. Gammans I am advised that many sound and all television receivers have circuit elements which radiate electro-magnetic waves when the receivers are in use. These radiations can be detected and located by direction-finding methods.

Following is a statement giving details of the working principles of the device which has been developed for detecting television receivers. The detection equipment utilises the induction magnetic field set up by the line scanning coils of the television receiver; this field contains strong harmonics of the fundamental line scanning frequency of 10.125 kc/s and these can be picked up by a sensitive receiver at distances of up to 100 ft. or more in many cases. Three horizontal loop aerials are employed; these are tuned to the second harmonic (20.25 kc/s) of the line frequency and are mounted on the roof of the detector van in an "L" formation. The outputs of the loops can be switched in turn to the input of the radio receiver and its audio output heard on a loudspeaker or indicated on an audio level meter. By noting the relative levels of the signals from the three loop aerials the operator can obtain a clear indication of whether the television receiver is in front of or behind, or to right or left, of the detection van. For example, when the van passes in front of a house on the same side of the road, containing a working television receiver, the indications obtained from the fore and aft loop aerials are equal, and that from the off-side loop aerial is weaker than that from the other two. The strength of the induction magnetic field varies inversely as the square of the distance and this fact enables receivers even in adjacent houses to be separately detected. The brick walls of ordinary houses have little or no effect on the induction magnetic field at the relatively low frequency (20.25 kc/s) used.

Great find Mate.

Bear in mind that at that time there was just one channel so the local oscillator signal wasn't needed to determine the channel being watched. Also, as with radio there were many "straight" (ie non-superhet) TV receivers without a local oscillator. As the detector vans detected a receiver's line output stage it is possible that the old advice to wrap the TV in metal foil might not have been such an old-wives' tale although I suspect that a pretty heavy metal screen would be needed to suppress the magnetic component.

Radio detection (radio licences were in force until 1971) did depend on detection of the local oscillator so again straight sets (and crystal sets) would be undetectable, although the long wire aerial needed for a crystal set would have been a bit of a giveaway.

Shady Pete