My point was that the fire chief could have detected COHb even if there was no CO on the plane. We both hold as suspect the suggestion that cabin CO caused the problem. But there are portable devices that can check for COHb - either from breath or from blood.
One passenger reported that pax from different parts of the plane actuated their call buttons at about the same time. This suggests that CO or not, there may have been a real (non-psychosomatic) cause for the illnesses.