The others were correct. It was the neighbors low battery smoke alarm.
I had high suspicion that it was that. But not hearing it thru neighbors door. And with how loud it was and the fact I could hear it thru my front door and not theirs. Made the hunt for it so much harder.
I'll add a related real life debugging tip I've seen play out at least twice in the real world:
If you can't get an alarm to shut-off or stop beeping, no matter what you try, then give serious thought to the possibility that there's a second alarm nearby which is the source of the noise.
About a year after I moved into my previous house, we heard that familiar chirping sound.
After 3 goddamn hours of searching, and having run to the store to buy 9-volts and replaced every goddamn battery in the house, we found a carbon monoxide alarm thrown INSIDE the return air duct of the HVAC unit...
Which is a reasonable place for one, as it will trigger from excess CO anywhere in the house when the HVAC is running. The problem is only obvious when you're not the original tenant who installed the thing.
I forget now. Was over a year ago now. It was just one that allowed me to zoom in on a specific point in the past 60 seconds of sound in real time to see an exact dB reading for a very specific and tiny spike.
To my ears it always sounded the same volume. And to some apps. It wasn't considered loud enough to be read in a meaningful way.
