Got all the kids out of the house despite the fact I knew it had to be a false alarm. The only CO2 producing appliance we have is a wood burning stove, and it was summer so hadn’t been used for months. But after calling the number on the alarm they urged us to call the fire department. So I called their non emergency number and explained the situation. They offered to send a truck.

An hour later after meeting some firemen - and my kids embarrassingly choosing to play our Switch instead of touring a fire truck - I called the CO2 alarm mfg again and they were apologizing and sending a replacement for a faulty alarm.

I think the alarm was about 2-3 years old and a stand alone unit.