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Hundreds of smartphone apps are monitoring users through their microphones
(www.the-independent.com)
This is a most excellent place for technology news and articles.
If it's trying to figure out if you're watching Stranger Things it can look for when you're stationary at home and just needs to record a few seconds at a time every few minutes. I don't know how the fingerprinting works. It might be able to run locally and not use a ton of power. We're talking Shazam, not full text transcription.