CO meter for sure, but a CO2 meter? It's actually a good idea to have CO alarms in your house if using natural gas powered appliances. However CO2 is only a concern if you're in a hermetically sealed environment like a submarine or space ship. I suppose it could be useful to check proper ventilation in the home, but normally you can just open a window.

Anyway the Earth has a carbon cycle, in other words it filters natural CO2 emissions through environmental processes. The problem is the amount added by industry is more than the natural carbon cycle can process. So levels are steadily increasing.

When we talk about zero carbon footprint we mean sources from industry like driving gasoline powered cars, generation of electricity, and production of consumer goods. A good amount already comes from natural processes like volcanos and erosion so we don't actually need a zero carbon footprint, just need it low enough to avoid overwhelming the natural cycle.

At a personal level it would be just about impossible to have a zero carbon footprint. If you had a solar and wind powered home off-grid and used it to charge an electric car you could be well below average. However any consumer goods you use put carbon in the air to produce them. Even if you went full native you'd still be putting carbon in the air burning wood and candles.