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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.
I can be absolutely certain no apps can access my mic in the background. Even when in the foreground, there is a hot-mic indicator.
Here's the misleading part:
Tap on one of those "while in use" apps. You'll see there is no option "allowed all the time." It doesn't exist.
"while in use" is top level, and changes to "while using the app" when you actually look at the permission. They really mean when the app is running. And many often run in the background.
Theoretically if mic gets accessed it should be lighting the indicator. Not sure if that's how it actually works though.