[LoisCampon]

The smoke detector in the old house I grew up in had a circular disk that had a radioactive sticker on it. I imagine the exit sings were similar. All I know is that smoke detector NEVER needed new batteries... it didn't even take batteries. It was also the scariest smoke detector I've ever heard. Not that cheesy, beeping sound they have now-a-days.

it was probably hard wired power wise, as for the sticker, smoke detectors use americanium as a ionizing source for the ionization chamber that sniffs out smoke