While visiting my mother-in-law, we wanted to get the smoke detector situation in her house up to par (this is in Massachusetts, we're from Europe, though). She has a natural gas fired forced air furnace in her basement; it blows warm air directly into various rooms instead of using radiators.
Above the furnace, we found the following fixture:
(Sorry for the overexposure, the lighting situation is not quite optimal there.)
To me, it looks a bit like the eviscerated remains of a smoke detector. What throws me off, though, is that it has two wire conduits leading to it: the bx conduit leads to a small box attached to the furnace, the smooth conduit leads directly to the breaker box. I'm used to smoke detectors running on an independent 9V battery, with no conduits. I'm also sure it's not just a broken light fixture, given the text on it says it's an "alarm". (The text also refers to detailed instructions on the inside of the base of the alarm, but the base is nowhere to be found.)
What is is thing and what would need to be done to repair it?