I inspected a home last week that had carbon monoxide (CO) pouring into the home when the furnace was running. Fortunately, the home had carbon monoxide detectors installed on each level of the home. Unfortunately, the detectors did not work due to their age. Fortunately, the home was vacant while it was being sold. Unfortunately, I had been inspecting the home for two hours before finding the large hole in the gas furnace exhaust vent. Fortunately, it was a warm day and the heat was not running.
Carbon monoxide detectors are only reliable for 7 years! You did not read the owner's manual did you. This home's CO detectors looked good, but they were made in 1977. They did not warn me when I ran a test of the gas furnace. The water heater also used the same air vent to exhaust its CO poison. Please check the date on your CO detectors and replace them when they exceed 7-years of age!