What might be causing horrible random beeping sound -- like smoke alarm -- HTC One A9?

[cathmary]

I have an HTC One A9, that is almost 2 years old. For first time today, it started randomly beeping a horrific smoke alarm-like noise. I don't use NTC so that is always off. I have shut off all Amber Alerts, etc. Right before the beeping sounded, I heard a vibrate sound but got no notifications otherwise from any app.

I run Malwarebytes and scan regularly for viruses -- nothing there. My Android version is 7.0, and I have the latest security patch (from 11/1/17)

The sound went away after I rebooted the phone. It sounded just like in this YouTube video https://www.youtube.com/watch?v=fjOAm2w4jTY (even though the phone in the video is a Samsung).

 

[SpookDroid]

All I can think of is either Amber alerts or alarms. Or a BT connection from a previously-paired device that's no longer authorized on your phone (like a BT stereo that was paired on a phone and then the phone was wiped)