There is a myth that carbon monoxide is heavier than air, and some people erroneously put their detectors closer to the ground. Carbon monoxide ("CO") is actually slightly lighter than air[1], not heavier. While the difference is so slight as not to usually make much of a difference (CO will typically distribute evenly through air), the production of CO in your home is often a result of inefficient combustion of a heating source in your home, like a malfunctioning furnace. As anyone who has been in a hot air balloon can tell you, warmer air rises.
Because CO is lighter and is usually coupled with rising warm air, it is generally recommended to put your CO detector closer to the ceiling, at least 16" away from any corners or side walls where the air in your home may stagnate. Nest, for instance, recommends that their Nest Protect Smoke+CO Alarm be placed in a central part of the ceiling outside one’s bedroom[2].
This answer is not definitive. When installing a new CO detector in your home, always ask a professional and consult your CO detector manufacturer's recommendation.