Mysterious noise keeps couple awake in their home - no source found!

[Mike Schroeder]

while this really isn't related, It is still slightly on track

I work at Home Depot, I was a cashier for a long time. I was working the Returns Desk on night and a lady came in and told me she wanted to return a Carbon Monoxide Detector, I ask her the standard is there something wrong with it? she tells me its broken because she plugs it in and it starts beeping. I looked at her for a second and asked her if maybe she had a Carbon Monoxide problem. She said she was not sure and left with the Carbon Monoxide Detector in hand