Earlier quoted context omitted.
CO is produced by bacteria in your gut, just like H2S.
The short version is more likely CO2, not CO, and the detector was probably fooled by methane. In more detail: I was motivated to google, and much like radioactivity or electric current its possible to measure "scary" things to low levels that don't usually matter, and according to a 1982 gastroenterology paper that I found, aerobic fecal fermentation will produce an irrelevent yet measurable extremely low CO output…
Classic :)
After digging through ebay and google images for awhile, I finally found one of the two different units that we used: http://www.equipcoservices.com/sales/rae/multirae_plus.html
That one actually has, at least what sounds like, a built in aquarium pump that actually draws air into the unit.
The other style we used seemed to be completely digital. It made no noise and the main unit was big and bulky. The sensor plugged into it with long wires and the sensor it self was just a big heavy plastic cube. On one side it had a screen and behind the screen was just what looked like metal. As if it could sense gases through metal or something, dunno.
And I don't know which model I was using at the time of my "experiment"