If you’re referring to Carbon Monoxide detectors [I recognize the OP made an error calling them CO2] commonly referred to as “smoke/CO”, then, in the US at least, smoke and Carbon Monoxide detectors are usually combined units. I’ve never found one to be separated like that. A common example is below. Carbon Monoxide is similar, if not lighter than common air density, so putting one on the floor wouldn’t make any real difference.