The hallway fire detector recently told us it scented carbon monoxide, but we hadn’t been doing any cooking or baking (in the kitchen two rooms away), the furnace (in the basement) hadn’t run for a few hours, and nothing else looked like it was on fire. I had recently replaced the alkaline batteries after a similar false alarm a few weeks earlier; it seems the detector failed after half a dozen years or so.
Tearing it apart revealed something resembling an 18650 lithium cell:
Which made no sense, given the circuitry.
A casual search shows a Figaro TGS5042 is actually a carbon monoxide sensor. I’m mildly surprised enough gas gets through the vents fast enough to produce an early alert:
I tore it apart to reveal a few droplets of whatever the electrolyte might be, so it hadn’t completely dried out.