I'm pretty sure a normal iOS app can't start a recording while it is not open (which also applies when the phone is locked). And even if they could, at least so

I'm pretty sure a normal iOS app can't start a recording while it is not open (which also applies when the phone is locked). And even if they could, at least some people would notice when there are billions of iOS devices around, that's a bit risky to just hope nobody sees the indicator.