Yes the CO meter has failed.
It is not a CO2 meter. CO2 is used in soft drinks, and it not to toxic. CO is what you need to detect, and it can come from the passing car exhaust (until the catalytic converter is warmed up a car engine can produce a lot of CO gas) or nearby generator, or other gas burning devices.
My buddies CO detector went off while in a campground and the neighbor's furnace came on, blowing CO and other furnace exhaust gas towards his trailer window.
Sometimes the CO meters can go off just because you are in town on a hot day. Don't know why the CO meters trip under such conditions, but it can happen with certain models.
Fred.