As I understand it, for practical purposes CO tends to mix with the air (because, although it is a different "weight" it is only very slightly different and so readily mixes with even slight air currents around the home).
As most modern boilers are a sealed system drawing combustion air in from outside and venting outside as well, in theory they shouldn't release CO into the room. I think an alarm should be positioned in the room where the boiler is but also in other areas, your living room and bedroom as well. CO alarms are now much cheaper than they ever were ( less than £20). Many now run on AA batteries rather than expensive PP9s. CO alarms have a limited life span after activation and cannot be reset.