I think that any receiver that uses the 'Superhet' principle has a 'Local Oscilator' built in to it. This generates a signal, abeit very weak, which could be detected posible by the on board unit. It is, or was, in my day as an electronics guy, the signal indeed that the TV licence van used to check not just the fact you had a TV, but even which channel you where watching at the time of monitoring.
FWIW
TJ