Carbon dioxide is measured by the room air sensor itself.

See Netatmo:

Square Carbon Monoxide Detector monitors fuel appliances (boiler, fireplace, cooker, gas range) by measuring in real time the level of carbon monoxide, an invisible and odourless gas that can be deadly.

If carbon monoxide is detected in your home, it alerts you with 85 dB and a real-time notification on your smartphone.

Correct :)