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masque wrote (edited )

There is an established tracking technique where certain apps will listen for inaudible tones embedded in broadcast media for various purposes (e.g. to tell which TV channels the user of a particular phone is watching, or to link a phone to a computer in order to build a unified tracking profile). Here's an article about that. I think they sometimes put these tracking beacons in physical locations as well (participating stores, etc.)

There was also a soccer league app that was caught using users' microphones to catch pubs streaming games without a license.

In both cases, the app is listening for a specific, known signal, which means that it doesn't need to transmit tons of audio data to a central server. But it's still definitely an example of your phone's microphone being used to gather information about you without your knowledge.

I've never seen any convincing evidence for the "I talked about needing a new umbrella and now I'm seeing ads for umbrellas, so my phone must be spying on me" idea. It usually seems like people just don't realize how much information they're leaking via other channels (mainly web tracking, location data, and credit card purchases, all of which can be very revealing).

That being said, I also don't buy the argument that this is categorically impossible because it would require transmitting too much audio data to a central server to be analyzed. I'm pretty sure you could run low-quality speech recognition locally, identify the most clearly spoken keywords, and just send that relatively small amount of data somewhere occasionally. The data quality wouldn't be very good on the individual level, but targeted ads and data mining are only really meant to work in aggregate anyways. I still haven't seen any evidence that this is actually happening, though.