I have special affection for carbon monoxide detectors, because one of them may have saved my life. When the alarm persisted, I called the fire department, and it found high levels of carbon monoxide due to a clogged chimney- heater connection. This illustrates that you don't want to assume the check up of your furnace will be properly done, and a carbon monoxide detector is a low-cost insurance policy. The standby is still the smoke detector. Almost all homes have at least one detector, but many do not have enough detectors.
Many people still think a detector at the top of the stairs solves all their problems.
But that approach can fall far short of minimum reasonable standards for fire safety. You should have detectors on every floor, as well as one outside and inside of every bedroom. Even for those who have detectors another one would probably be useful and might even save lives. There's also a good chance that a mounted detector may be obsolete and in need of replacement. Most owner's manuals say replace every ten years.