It only happens in Messages.
I’ll be typing a message, hit send, then all of a sudden, Messages starts recording audio. It doesn’t happen often, so I never put in the energy to figure out what was going on.
It happened to me again yesterday, and I posted about it on Twitter. To my great relief, this has happened to other people as well (so I’m not that crazy) and it turns out this is a feature, not a bug.
To see this for yourself, go into Messages, pick a specific thread so you see the text editing field at the bottom of the screen (with the camera icon on the left and the mic icon on the right).
Now lift the phone to your ear, as if you were making a call. You’ll hear a soft chime and Messages will start recording audio. Bring the phone away from your ear and the recording will end, automatically. If you like, you can hit send and share your audio message.
What caused the audio to start recording was the proximity sensor (it’s at the top end of the phone) detecting that you’ve raised the phone to your ear. But, key to this process, the phone must also detect a specific twisting motion to enable recording.
Try holding the phone upright, wait a few seconds, then bring it to your ear, without twisting the phone. Recording will not start. It’s the combination of the twisting motion and proximity to your ear. You can also make recording start by doing that same twisting motion and bringing your phone up to your hand, or even your coffee cup (the proximity sensor uses light, not heat, so any big enough object will trigger it).
Apparently, there are times when I inadvertently make that twisting motion and cover the proximity sensor. This causes Messages to start recording.
Turns out, you can disable this behavior. Go to Settings > Messages and disable Raise to Listen. This setting has been around a long time. I just never put it together with the seemingly random recording. I did find the whole thing fascinating, though.
Big thanks to Carlos Oliveira for helping me figure this out.