I had an issue a while back that fits in here as a FYI (For Your Information).
My carbon monoxide detector was occasionally going off. Couldn't figure out why. No engine running - pulled the propane tank off the boat as a propane leak can set it off - changed the detector - it still kept going off occasionally.
What I found was the batteries were gasing excessively. Discovered this when the batteries wouldn't last overnight and I replaced them.
So, they should bubble - gas - but there is a limit as to how much they should gas.
I bring this up in case someone else might have a problem with the carbon monoxide detector and can't find anything. Check the batteries.