I'll be honest and admit that hadn't even crossed my mind.
Although we have CO alarms in the van (by law when manufactured) and at home our boilers - probably like most - are balanced flue which are unlikely to leak CO. The dodgy things are those with an open flame such as gas fires, ovens, and some water heaters. The van does have a fire and oven but we only use the oven top rings and poor combustion there would be visually obvious and the fire not at all. I don't know the details of what went wrong with the bloke at works parents.