I've got an Aranet4 and started tracking CO2 and submitting it to IndoorCO2Map (via https://whn.global/indoor-co2-map-co2-monitoring-and-data-co...), but after a short period of time, there aren't really any surprises. Carrying a CO2 detector everywhere seems like mostly a way to give yourself anxiety.

If it's crowded now, or was recently, the CO2 is going to be high. If the building is old, or low volume, the effect will be worse.