Then probably both. If it beeps downstairs, do you wake up upstairs? Same problem as with the smoke detectors. Solution: Networked devices.
I just read that a power generator was said to be responsible for the CO emissions – although this sounds only semi-official so far. It was operated in the cabin... if that is true, the owner, if he installed the generator there, will probably find himself in court soon. Unfortunately, this incident then falls into the category of "lack of common sense."
A few years ago, there was an incident at Wacken Open Air. A visitor lay down to sleep on a trailer after a night of heavy partying. Under the tarpaulin, under which the power generator also stood... you can imagine the rest.