We used to have a fancy ass fire alarm system in the 90s and early 2000s. At a couple of points in the house there were detectors, each with a battery, power connected and connected to each other. If one of them detected smoke or heat, they would all start beeping. They would flash every 10 secs to let you know it is in sync, but they would also emit a small beep every 10 mins or so. They never ran out of power since they were connected to mains voltage with a battery backup. The battery was rated to be replaced every 10 years, but I think it was one of those that basically went on forever.
I got used to it after a while, but every now and again I would start noticing the beeps and could not stop noticing them. My dad was proud of them, he paid a lot of money to get them and get them installed.