The minimum level of CO2 that exists in the Earth’s atmosphere is slightly above 400ppm. This is the observed outside value of CO2. (Source: https://www.scientificamerican.com/article/earth-s-co2-passes-the-400-ppm-threshold-maybe-permanently/).
Our CO2 sensor assumes 400ppm to be the lowest background level as that's the outdoor CO2 baseline level. Humans inside building tends to be the major source of CO2. When a building is unoccupied for 4 to 8 hours, CO2 levels tends to drop to outside background levels.