We respond to CO alarm activations for the same reason we respond to fire alarms.
When we respond to an AFA, we expect to find a fire when we get there. When we respond to a CO alarm, we expect to find CO when we get there.
To properly investigate a CO alarm activation takes some time. ALL potential sources must be checked.
Most CO alarms activate based on time vs. exposure. A low level of CO over a longer period of time will cause the alarm to activate, as will a higher exposure over a short period.
Because CO is odorless and cannot be seen, it is even more important to respond and investigate using the proper equipment. I prefer at least a 3 gas meter so you can verify CO, O2, and Combustible gas readings.
We can limit our liability by being properly trained to investigate these incidents and taking our time to investigate all potential sources. Not responding does not remove liability as I am fairly certain most fire departments investigate these calls for service.