There was a mystery beep at my parents’ place for years. It was so rare that you couldn’t feasibly track it down (walk a little way in the direction you think you heard it from, wait for the next beep, etc.) It happened like every half hour or something. Maddening.
Turned out it was the UPS on the thing that connected the copper phone lines in the house to the fiber optic cable running out to the street. I only figured it out because I happened to be doing a load of laundry in the basement next to where the box was, and looking at the box when it beeped, and a little LED flashed in sync with the beep.
