May I suggest flipping the breaker for those hard wired detectors the next night it’s warm enough to sleep w/ the windows open?
Some birds have taken to imitating the sound of electronics.
Or, maybe, a truck outside, backing up?
Ugh. My mystery beep was the battery dying in a food thermometer in a cutlery drawer.
Poor Montresor has been signalling you in Morse Code nightly with his fading watch alarm and you’ve left him walled up all this time?!
This.
I assume you saw these posts and don’t have Fios, Chefguy?
My “What’s Beeping?” story turned out to be one of my kid’s Furbies! I was ready to rip the sheet rock off the wall to find it! The batteries were running low and I guess it was giving a warning.
So… I’m going to suggest you check any kids toys; if that’s the sort of thing you have around your house. Good Luck!
I’ll bet you dollars to donuts it’s the CO detector (yes I read the OP). They chirp three times when failing no where near as loud as it’s actual alarm. Humor me and take the batteries out and see if it continues. While you are at it check the date on it. They only have about a 10 year life.
Correct. Also, no kids in the house. Also, CO detector is right next to the bed, so we would have figured it out right away.
And no beeping last night.
Prepare for beeping twice tonight.
Do you have any enemies? Perhaps someone ‘gifted’ you with an Annoy-a-tron…
No beeping for the past five nights. Whatever it was either died or was somehow remedied.
I love a happy ending. 
beep…beep
At the place I stay in Barbados every winter one of the BRs that my son was about to come and sleep in had such a beep. Every 30 seconds and we could not hear where it was coming from. Finally the electrician we got noticed some locked cabinets. Fortunately, the housekeeper had keys. Sure enough there was a dying smoke detector inside. Utter stupidity.