just a small post and end of season but Carbon monoxide detectors on boats - they do work.
A recent trip to Poole we ran the engine with the covers up but unzipped- cant remember why probably just a quick battery charge, anyway - after 5 mins the cabin CO detector went nuts. wall mounted half way up the bulkhead wall just inside the door way.
No harm to us as we were outside drinking tea and I'm fully aware of not running the engines, covers up and being below decks so no big issue but it proves there is a need and they should be fitted. Hopefully you all have one. or will have next season.