The local adopting agency (usually the state, but sometimes the county or municipality) adopts the model code, then will add to and/or modify certain sections of the model document. Some states will also have the model code printing press print a specific state modified version of the building code. (Florida, California, Massachusetts and Connecticut come to mind)
The model building codes do not require CO detectors at this time (2003 IBC or IRC) I haven't checked the 2006. The NEC never has required fire detectors/alarms or CO detectors/alarms. This always has been a function of the building code.
Get on line, check out your state for the codes adopted, and their modifications of the model code.
