Interesting... Unless you have a very visible / audible digital CO2 monitor, if you use of those small disposable ones then I guess you need to incorporate it into your scan. I have a small supply of those disposable ones and it dawned on me that I hardly check it when I get in the plane, let alone during flight. Maybe it's because it is stuck out of the way on a side window, so I will move it more centrally and try and incorporate it into my scan.