carbon monoxide detector going off (update)

Check that the water heater flue is clean and the flame looks good and the burner is adjusted properly. Incomplete combustion results in the production of much, much more carbon monoxide than proper combustion. My first guess would be insect nests or something similar in the burner or flue assembly. My guess may be completely wrong, of course; it would not be the first, and probably not the last time!

I would doubt that your detector is on the fritz since it correlates very closely to using a gas appliance. Figure out what's happening with the burner, and be thankful you had a working alarm to alert you to a potentially quite dangerous situation.