That is one of the reasons that CO detectors aren't in the building code...there is no good standard to reference. The Utah Chapter of ICC submitted it to the ICC conference, but it was rejected for that reason. Utah requires them in homes, however.
As far as elevation goes, CO has a vapor density of 1.0, which is the same as air. This means that it neither sinks nor floats in air. I would say put it anywhere.
