NFR Murphy's Law

[Holders]

I tested each device after reinstalling with new batteries.  Press the test button which cycled the beeping throughout the house.  Did all that.  And STILL the beeping continued.  All of them are connected to in-ceiling wiring via a 3-pin plug.  You unplug the pin and you can remove the device (which I did).  Made a number of calls and learned the detectors have a usable lifespan.

In some situations there's a certification period by which they have to be replaced, faulty or not.