Check your exact model. Each one have different specifications. The truth is that CO detection has improved considerably in the last 30 years, and the sensors no longer work by "soaking in" X amount of CO, so many are perfectly fine being installed on the ceilling.

I know I installed brand new detectors, and they detected output of CO by our old furnace. Forced air is pretty turbulent, so it's not like all the CO is just gonna flow consistently to one place. Think of it like having a pitcher of oil and water perfectly separated. Pouring it into a new container will not maintain that perfect separation. Same thing for air, but the effect is even more pronounced.

And CO is lighter than air, so you would expect it to rise eventually anyway.