Smoke is hot and rises -- so smoke detectors are placed on the ceiling or close to the ceiling. Carbon Monoxide (CO) is a gas that is about the same weight as regular air -- so it does not tend to either rise or fall significantly in the house. So CO detectors can be placed anywhere; in fact this is why they often come with plugs sticking out the back. They can simply be plugged into any outlet.
But you want to avoid areas where there is a strong breeze, because the gas will not stay in the detector long enough to get a reading -- so don't put them in walkways. You also want to avoid areas where no air doesn?t move at all, like in stuffy corners, because the CO will take a long time to get there. It is recommended to have one near the door to the garage (but not in the breezeway), and one near the furnace area. And there should be one in or near bedrooms, where we need the alert more than anywhere else because we will be sleeping and not aware of the symptoms of CO poisoning. Drowsiness is the primary symptom of a CO problem, so we just don't know there is a problem until it is very serious. A CO detector catches problems much faster than we do.
Don't forget that all detectors, CO as well as smoke, only last so long. Look carefully and you will see date limits on most of them nowadays, which gives some indication as to when the detecting mechanisms may stop working reliably. The test buttons test the electronic circuits, not the detector mechanisms. If yours has been there for more than 5 years, it is best to replace it.