To reiterate Tengu’s point, carbon monoxide detectors are different than smoke detectors. The sensors have a shorter life span and the detector will continue to beep (indicating pending failure of the sensor, not low batteries) even if you replace the batteries.
Check your dishwasher. When mine doesn’t complete the entire cycle it beeps until is start over and runs another batch of dishes.
Had something like that happen once. It turned out to be the battery on the Verizon FiOS box.
Beeping mystery solved! Descent into madness averted! 
My spouse kept swearing it was my laptop–he claimed the sound was coming from that general area. I told him it wasn’t, but just to humor him I unplugged the laptop. Still beeping. Went back to watching TV, then headed upstairs to get ready for bed. Next thing I know he’s knocking on my bathroom door. “I found it!”
“Well, what is it?”
“You have to see this.”
So he leads me downstairs, to the table where the laptop is. Under a pile of papers is the module from my cheap exercise bike, which just hooks onto the front of the bike with velcro (and falls off all the time, which is why it was on the table buried under papers instead of on the bike). He points triumphantly at it. “Do you hear any more beeping?”
I listened, hardly daring to hope. Time passes. Silence.
Hooray! The Mysterious Beeping Noise has been vanquished!
Thanks to everyone for all the suggestions, and apologies for not providing a proper spiral into the depths of insanity. I’ll try harder next time. 
I’m confused. Why was the module beeping? And did you just turn it off to get rid of the noise?
Probably due to low batteries. It came with crappy cheap batteries. I had no idea it beeped, but given that he took them out and chucked them and the beeping stopped, I’m going to assume that was the cause. Either that or it was the world’s biggest coincidence.
When I was living in the desert I’d occasionally wake up at night to the sound of beeping. There was a high tone and a low tone. Being the early-'80s there weren’t a lot of gadgets that beeped. The sounds were definitely coming from outside, and I went out to investigate a couple of times. It seemed to be coming from everywhere and nowhere. I never did find out what it was.
I told you so!
That is pure evil.
:smack: Can’t believe I missed seeing this before posting the same link earlier today. Congrats on finding the culprit!
I about drove my nephew out of his mind last Halloween when I put the Eviltron variant (spooky noises) into his bedroom.
Picked up this thread as I also have had an intermittent beeping in the house for approx. 3-4 months. I could put up with it but it really annoyed my dog, where she would head to the bedroom & lie under the bed for hours! At first I thought it was the burglary alarm. I replaced the battery in the panel, but the beeping continued. Changed the batteries in the smoke detectors, no joy! Changed the battery in the CO moniter - still more beeping :mad:
Got onto this thread & found the comments interesting. So to cut a very long story short, the beeping was a blood pressure monitor my husband threw in the bottom of a cupboard. :smack:
Did you use the blood pressure monitor to see how much your blood pressure had gone up from the incessant beeping? 
Couldn’t.
The batteries were too dead.
Do zombies have blood pressure?
This might sound crazy but my wife asked if I heard a beeping sound? At first I didn’t then I walked around and heard it faintly. Exploring a little more I could hear it coming from our garage. The beeping sound was a beep beep beep sound always in a pattern of three beeps. At first I thought it was coming from under the hood of one our cars. It wasn’t but it then seemed to be beeping from a shelf on the side of the garage wall. My wife came out and asked what it was? I didn’t know and still have no idea. The sound seemed to be coming from a particular place on the top shelf where I keep seldom used power tools. I had to get a step stool to take a look up that high didn’t see anything but the plastic cases holding the different power tools. I haven’t used anything on that shelf in months and perhaps years, there are no electrical sockets, no battery powered anything. The beeping moved left and right and back left. I asked my wife if she was playing a trick on me and she said no way! I repositioned the step stool and went to where the sound seemed to be emanating. I moved a plastic case holding a planer that I haven’t used in years. The beep beep beep sound stopped but there was nothing there! I have no idea what made or created this sound and have gone out to check twice to see if I could find anything but there just isn’t anything there. It was 100% definitely coming from in the garage at that spot and I’m relieved my wife witnessed or heard the same thing I did. I have no idea what it could have been but it lasted about five minutes. I am completely baffled. Has anybody else had this happen?
The batteries could still be dead , my daughter got package of batteries and some of them were dead . She had to return the package for another package. I am thinking the beeping could be from outside too. Maybe you could ask your neighbors if they’re hearing a beeping sound too.
The beep beep beep was specifically coming from that area in our attached garage. Our neighbor’s home is too far away to have a sound and it was raining which would muffle any outdoor sounds anyway. Here it is the next day and I’m still baffled at what that sound could have been. I appreciate you responding and offering your thoughts though.
My son-in-law was here when the beeping began again and quickly found the reason. On a lower shelf was an old smoke detector in a box of garden gloves that I hadn’t looked in. It’s a relief to know it’s solved and I feel a little embarrassed to not have looked in that box. I hope that others who have mysterious beeping sounds are able to solve their mystery sounds as easily as this. To all who have read any of this post - I hope you have a great day!
There was an episode of The White Rabbit Project (the Mythbusters build team’s crappy new show) where they talked about how lousy standard beeping is for position detection. The human ear isn’t built to locate the source of such beepings.
OTOH, they showed that white noise is far easier for finding the origin.