The detector vans were real, and they weren’t a bluff, but the tech they used wasn’t some high tech signal detector. The secret is that they just pointed a parabolic microphone (possibly a laser microphone at a later time) at your window and listened for the audio. The operator would flip through TV channels in his van and try to match the audio from your house to the audio from a currently broadcasting TV station. That was sufficient to determine if someone was watching broadcast tv or not.