The last thing it could be is an actual alarm from the clock app, that had that type of sound selected for it.
If it was not an alarm, then I'm at a loss as to what it could be.
There's no other explanation for it that I can fathom.
That's what I checked first. I went through every alarm sound and there is not even one that sounds like what it did and can't get it as loud as what it was. I was in our shop and you could hear it in the house.
It may have been an alert:
About emergency and AMBER alerts on your iPhone - Apple Support
I wasn't on any app or game. My phone was sitting on the table. That's why it took us a while
To even realize it was my phone and not our smoke detectors or alarm system
A bit of searching produced this:
Looks to be an ad playing the sound. Probably from a game or App.
It was nothing like that sound. It was literally like a smoke detector that was a constant and VERY loud beeping.
My phone just all of the sudden started making a loud beeping that we thought it was our smoke detector
