We have a carbon monoxide detector in our P275. It is located in the cabin at the opposite end to the hatch. Although I can smell diesel fumes in the cockpit in certain wind conditions the alarm never goes off. Your engine and detector are much closer than ours. Let’s hear from other P21 and P235 owners
To check the Alarm was Genuine i brought in a second detector from Home which was a different make.
The tailwind was such that there were waves on my Gin and Tonic, but since both detectors went off were they detecting not Carbon Monoxide but Carbon Dioxide, possibly from the Tonic Water ?
Stay safe! John