I just wanted to come here and add to the conversation because this thread saved me. I had a wired+battery alarm chirping away in the kitchen, chipping away at my sanity and scaring my cats. I took out the battery. I left the unit disconnected. I turned off the smoke alarm ring at the fuse box. I turned off the kitchen ring at the fuse box. I tripped the entire lot of fuses and plunged the house into darkness -- and still the occulted thing chirped! I tell you, I was this close to starting to pull out wires from the ceiling. 

But then this thread mentioned that if this paranormal activity was happening then it couldn't possibly be the alarm, but perhaps something nearby. 

It was indeed a nearby carbon monoxide alarm that was the sneaky sanity sapper. It would be great if they didn't sound quite so smoke-alarm-esque.

It also explains why I found that the chirp sounded strangely louder when I was near the CO alarm LOL. 

Thank you 🙏 

I climbed ladders and exchanged units and batteries for the last 48 hours chasing the beeping noise then I consulted this site.

As forecast by the experts it was a small CO2 alarm we had completely forgotten about and was out of sight.

PROBLEM SOLVED!….thanks.

Thanks Nick, it is surprising how many people blame the smoke alarm when it is another device tucked away in a drawer or on top of the cupboard.

But I have to correct you, you found a CO alarm not a CO2 alarm. (The two are NOT the same)

CO is carbon monoxide, CO2 is carbon dioxide, It's what you get in CO2 fire extinguishers as shown below.