Facebook has patented a system that can activate people's devices in order to study what TV programmes and adverts they are watching, but it says it will never use it.
Facebook's patent application describes a system that would embed what it calls "audio fingerprints" on TV shows and ads, that would be inaudible to human ears.
These audio fingerprints could then trigger microphones in the room, such as on phones or tablets, to begin recording the "ambient sound" and allow Facebook to collect data on what the user was watching.
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The patent, first reported by the New York Times, has been compared to the music-identifying app Shazam.
Facebook head of intellectual property Allen Lo has downplayed the patent, saying in a statement reported by The Guardian that the technology "has not been included in any of our products, and never will be".
"It is common practice to file patents to prevent aggression from other companies. Because of this, patents tend to focus on future-looking technology that is often speculative in nature and could be commercialised by other companies," Mr Lo said in the statement.
Despite the resaurrance from Facebook, a privacy expert has raised the alarm about the system and the potential for people's private conversations to be caught up in the audio recording and show which groups of people were meeting up.
Electronic Frontier Foundation senior staff technologist William Budington told The Guardian it was "extremely disconcerting".
It's a popular conspiracy theory that Facebook already uses microphones to listen to people's conversations in order to target adverts to users, but the company has consistently denied the claim.
During a hearing at the US senate in April, Facebook CEO Mark Zuckerberg was asked whether it was true.
"No. Let me be clear on this: You're talking about this conspiracy theory that gets passed around that we listen to what's going on on your microphone and use that for ads. We don't do that," he said.