Peter: I would think it unlikely that exhaust fumes are being drawn in by the airflow as this possibility would surely have shown up during initial flight testing (wool tufts etc).
My money's on the muff passing fumes through a pinhole with the engine working hard on T/O & climb or accumulating in the engine compartment at idle because there's no dissipating flow through the cowling.
You might find
this link interesting regarding domestic CO levels. I'd imagine continuous zero readings would be preferable as far as flying is concerned but aeromedical aspects are outside my field.
Perhaps you should ask your AMO when the muff pressure test is next due. It used to be on the Annual Inspection but I don't know how it stands nowadays.