Typical CO alarms only go off when there is a level of gas that is an imminent danger. A lower level of gas could be present and go totally undetected, but present an ongoing health risk. There are special CO monitors that are capable of detecting and alerting down to 5ppm but they are rarely installed in homes.

Well it sounds like the bulk of the pollution was methane and hydrocarbons, maybe some co alarms don’t do well in heavy background gas like that.