We have always has a few CO detectors. About 10 years ago the one in the basement nearest to the furnace started just chirping at us intermittently every few days. I thought the detector had gone bad, but had an old friend in the furnace business come for a look. Sure enough he discovered a crack in the 25 year old furnace internals. Yikes!!
Exact same thing happened in my new across the street neighbor's house. The repair guys were there all day trying to find the problem. It looked to me like they just kept throwing new parts (and a complete new stack from basement to 2-story roof) at it until the sniffers stopped going off.
Same as many auto repair shops will do.