carbon monoxide is less dense than air - according to wikipedia, density at room temperature is 1.15 kg/m³ for CO vs 1.275 kg/m³ for dry air.

The nest protect retails for US$129.00 and has a service lifetime of 7 years.

I recently checked all the smoke detectors at my parents house and found that they were all well past their 10-year replace-by date. (I replaced them, but not with Nest smoke detectors).