400ppm with stagnant wind blowing co2 from volcano directly into sensor.. 

The NOAA Mona Loa observatory is located on the side of active mona loa volcano. Adjacent, is Kilauea, which releases 300 tonnes per day of CO2 and other gasses. On the day of the report, winds were stagnant in the area and blowing the entire gas plumes of the active volcanoes directly into the NOAA Observatory sensor. It is rare that both low and high altitude winds go in this direction, happens only a few times per year, and on this occasion it continued for a week. This false report is the equivalent of sticking the sensor in the volcano and noticing there is gas coming out.