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.
How weird, I’ve never seen that configuration. Today I learned. I just wanted to add that in case anyone read that and thought their living space was unsafe with a combination detector. The EPA says, if they are CO specific, to put them about eye level from the floor, or on the ceiling.
https://www.adt.com/resources/carbon-monoxide-detector-placement#:~:text=Carbon monoxide is lighter than,them on the ceiling%2C too.