Question

Can carbon monoxide leak (from a fireplace, barbeque, water heater)?

Answer 1

yes. CO is produced by barbeques, fireplaces, and water heaters (if the water heaters are fired--but not if they are heated electrically). So you want to barbeque outside and you want to keep the flues for the fireplace and water heater (that's what vents the CO to the outside) clean and working very well. They can still leak of course.