Really! Was it a kind of barstool conversation perhaps. Because, you know "listening devices" need to be powered on to listen to "encrypted signals".
A low-power standby mode, in which the only thing powered is a simple receiver without a lot of smarts, whose only job is to listen for a specific signal and then, when it hears it, turn on the rest of the circuitry (which includes fancy signal processing, data analysis and storage, reporting, two-way comms, etc.) is not at all a crazy design pattern; it's used all over the place.