Rojo - my understanding of how the 'old type' TV detector vans used to work and thus know you had a TV is this: People in the van will fire an RF signal at your property. If you have a TV, the local oscillator on the TV will pick up the signal and re-radiate it. As you know, the local oscillator is at a pre-set frequency, and if your TV re-radiates the signal - hey presto, you have a TV. It doesn't even need to be on, they are just looking for a re-radiated signal. The best way to stop it would have been to shield the local oscillator, but that's another story. It's not rocket science but I can understand how they would like to keep it from becoming public knowledge.
How it works with digital - no idea.