I don't know, I think in any normally populated area there would be so much crap that it would not be possible unless there was some guy on your roof next to the antenna
plugging a antenna into a 'power signal' in the power supply through a coupling capacitor is I think a bit more powerful then the natural emissions? are you sure thats a good comparison?
not to mention faulty equipment and crazy wiring making the signals go everywhere? Especially if its tuned to regions that are not empty.
Now, if a spy has a radio tuned in some RF 'dead zone' waiting for a signal, in some isolated place, that might be somewhat easier to pick up (waiting for numbers station broadcast), but if your listening in the normal 'fm band', in a normal populated area, with a normally hook up radio, I think it might be a bit of security theater going on.
and I think if it was that easy, it would be exploited and known, i.e. burglars known to listen to RF signals to wait till someone falls asleep before doing some 'cat work'. Especially when you had battery powered radios, you basically know nobody would leave that on after they went to sleep, because its expensive, and also annoying to replace batteries
I would fully expect that getting a job as a " RADIO DETECTOR VAN DRIVER" would be a appealing position for any half decent criminal. Lights out, radios dead, whats that give you like a 50% lower chance of having a close encounter with a occupant during a burglary? You can even get rid of that nagging urge to revisit the crime scene to issue a citation later. And they basically let you case the place at night, regularly, with zero suspicion, so you can learn the exact habits of targets. I think this would give you real life 'criminal superpowers' And you can probably weasel your way out of a arrest or lessen charges because you can say you got job frustration about 'in determinant signals' or some other crap that possessed you to enter someones property.
I suspect in practice this radio van did not work very well.