My friend told me thought this was happening a few years ago, and mentioned he thought he could hear the mic turning on in FB messenger. I thought this was bullshit, because why would you be able to hear the *microphone* turn on?
Well,, I tested it on my Galaxy S5...
I put my ear up to the microphone, and turned on the stock voice recorder app. Sure enough, I was able to hear a bit of static. It was probably coming from the speaker, and I don't know why it occurs at all, but it certainly did. I tried again with calling somebody, and I heard the same distinctive static sound before the call connected (so it wasn't general phone static). I tried again with the video recorder, and heard the exact same static. It's sort of like a fuzzy click sound followed by a very faint static buzzing, and I could only hear it in very quiet rooms.
And then, I tried opening the messenger app... as soon as I either (A): opened OR received a new message (B) sent a message or (C) opened the app, I could hear the exact same distinctive static turn on for just a few seconds, maybe for between 5 - 15 seconds. This confirmed to me that the messenger app absolutely records audio.
Again, I have no idea why the microphone would make a sound, unless it's some sort of feedback loop between it and the speaker. But I verified that very distinctive sound only occured when the mic should be on, and discovered the sound when using Facebook messenger. I don't remember trying the normal FB app but I suspect it does the same. Other people, please try this experiment and tell me what you find.