This photo is of a carbon monoxide detector installed in the hallway of a home built in the 70's. One of the reasons we have building codes is for safety. They are written re-actively stemming out of past situations. In the past people were sickened or killed by carbon monoxide and the building codes were updated to address this issue for everyone's protection. It is always recommended to update your home to meet the latest safety standards. If you have gas or propane piped into your home for a furnace, water heater, fireplace, or even an simply an attached garage, and do not have a carbon monoxide detector then you are putting yourself and your family at risk of harm from carbon monoxide poisoning.
553.885 Carbon monoxide alarm required.—
(1) Every separate building or addition to an existing building... constructed on or after July 1, 2008, and having a fossil-fuel-burning heater or appliance, a fireplace, an attached garage, or other feature, fixture, or element that emits carbon monoxide as a byproduct of combustion shall have an approved operational carbon monoxide alarm installed within 10 feet of each room used for sleeping purposes in the new building or addition, or at such other locations as required by the Florida Building Code. The requirements of this subsection may be satisfied with the installation of a hard-wired or battery-powered carbon monoxide alarm or a hard-wired or battery-powered combination carbon monoxide and smoke alarm.
Below is a photo of a combination smoke and carbon monoxide detector in a newer home. This detector is installed in a home without any gas or propane - but it has an attached garage. The carbon monoxide gas would emit from the exhaust of an automobile or other type of engine that was left running in the garage, and somehow made it into the living space.
For those that want to read further into this topic - here is a link to some in depth information: