Carbon monoxide has no smell or colour. On a boat it comes from engines, generators and fuel-burning heaters and cookers, and it collects in cabins where people sleep. A detector is the only warning.
ABYC A-24 covers CO detectors. In ABYC's own summary, detectors must monitor habitable spaces, meaning spaces used for sleeping, cooking, eating, navigating or steering, and washing or the heads, and must be tested to UL 2034. ABYC also lists where not to mount them, including beside hatches and ventilation openings where the air is diluted.
What a surveyor checks
Walk through each habitable space and look for a detector. Read its label: the standard it meets, marine use and the replacement date. Press the test button. Then look at the sources: generator and engine exhausts, heater flues and anything that could let exhaust into the cabin.
In the report
"Forward and aft cabins, no CO detector fitted. Boat has a generator and a diesel heater. Fit marine CO detectors to ABYC A-24 in each sleeping cabin before the boat is used overnight." See no CO alarm.