"CO has a molecular weight of 28 g/mol and a density in air of 1.145 g/l.

The density of air is 1.2 g/l. (Air is mostly N2, which is also almost exactly 28g/mol.)

So the carbon monoxide is very slightly lighter than air. It does tend to rise, but mostly it mixes in with the air, and any currents in the air at all will bring some of it down to where the detector can detect it."