That always gets my dander up. Either someone removed the batteries for use in another device, or the batteries were getting low, started that slow annoying warning beep, and somebody pulled the batteries and never replaced them.
Might not have been true in this case, but too many people have died over the lack of a few dollars worth of 9V batteries.
Probably the latter. And it really appalls me that people think they can afford to squirt out half a dozen babies, when they can't afford a couple of dollars a year to keep good batteries in their smoke detectors.