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Hundreds of smartphone apps are monitoring users through their microphones
(www.the-independent.com)
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The apps need permission to the microphone for that and in background it would drain the battery and constantly show the microphone is accessed unless you are using an old android version. It is likely easy though to eavesdrop when the app is opened and using microphone is an expected functionality, and to pick up keywords and the sounds emitted from other sources to better know location and social graphs without GPS access.