Carbon monoxide ...
Wierd. Yesterday I took some friends flying. We did a couple of hours with no issues. On the way home they wanted some photos of their house, so I flew a few fairly heavily-banked orbits over the required spot.
I always carry a pretty sensitive CO detector on board (not the little paper ones, but an electronic one), and all of a sudden it went mad. It continued for some time after I'd pulled out of the orbits and continued en route.
Anyone any theories ? Ingesting my own exhaust, perhaps ?