My iPhone let out an ear-piercing siren-like alarm?

It was about 6:30 in the morning at this point. I had no alarms set, and none of my push notifications would ever do this. But out of nowhere my phone let out this piercingly loud sound

The only way I can describe the sound is this: y'know how whenever you're listening to the radio or tv and the emergency alert service interrupts what you're listening to (either to do a test or warn you of hazardous weather) and it goes "beeeeep.....beeeeep....beeeeep....beep...beep...beep" and then you hear a high pitched "EEEERRRRR" sound?

Well, it sounded just like that high-pitched sound. And it sounded off in second long beeps (whereas a regular alarm sounds almost twice every second). And my phone vibrated even though I had it on do not disturb mode. There was some a text alert on my screen, and it filled the entire screen, but I was so tired and so freaked out by the sudden noise that I just unlocked my phone to the home screen to get the noise to stop

I know it wasn't an Amber alert because when I get one of those, it sounds off as a regular tri-tone alert and has a tiny amount of alert text

Does anyone know what this could have been? My iPhone is a 4s on the latest update