Yes.

If you still have the detector, I would very much like to see it tested to see if it actually still works. Detectors this old usually use a solid state tin oxide semiconductor based detector that have extremely long lifetimes, rather than the newer mandated fuel cell types which dry out after 10 years or so. If you can, do a video placing the detector in a closed container, like a pot with a glass lid or on a counter under an upturned glass or plastic bowl, with a piece of burning incense inside. Leave it there for at least an hour to account for the inbuilt delay time on some detectors.

Your correct

I read that CO mixes with other air so is throughout. If the monitor is for gas as well as CO, it should be placed high for natural gas and low for propane gas.

This device detects carbon monoxide (CO), not carbon dioxide (CO2).

This product is not a carbon dioxide (CO2) detector. Regarding carbon monoxide (CO): Although CO is roughly the same weight as air (CO's specific gravity is 0.9657 [EPA]; the National Resource Council lists the specific gravity of air as one), it may be contained in warm air coming from combustion appliances such as home heating equipment. If this is the case, carbon monoxide will rise with the warmer air.

There is a slot on the back that says "Slide to Open." Slide that open and that is where you put in 2 AA batteries. Thanks for watching!

If you're plugging it into a wall of a room, it should be at least 6 inches from the ceiling or 6 inches from the floor. It also should be at least 5 feet from any fire-burning appliance. In this video, I was placing it in the kitchen so I wanted to ensure that it was 5 feet above the floor so that it covered all necessary standards to ensure effective detection. Hope that helps answer your question, thanks for watching the video, and all the best in keeping your home safe!

Sumukh Torgalkar thank you!