If you're reading this and you don't have carbon monoxide detectors in your home, please go buy some. They are generally integrated in modern smoke detectors, and the standalone units are pretty cheap.
I'd recommend having one in the garage if you have a gas water heater. But you should definitely have them in the house no matter what.
> They are generally integrated in modern smoke detectors
No they are not. Most smoke detectors do not have carbon monoxide detection built in. You can buy combination units but you should not assume a smoke detector has carbon monoxide detection just because it’s new. Verify that the unit you purchase provides detection of both of you want both in one.
And replace your detectors if they’re 10 years old (or don’t have a date, in which case they’re way older than 10 years).
Source: Old heater in a caravan broke, bit too much fuel in it during ignition caused a small explosion that blew the chimney (?) pipe off. We got away with a collective headache at least.
I'd recommend having one in the garage if you have a gas water heater. But you should definitely have them in the house no matter what.