I think the methane thing is a bit of a red herring TBH. It does exist, but I doubt it is related to this incident.
From memory (ie. don't quote me on it, but its along these lines), when they built the Haunted House, it was later discovered that the site contained a very small natural methane source, which sometimes causes a weak build up of the gas. If you're ever riding Duel and notice you can see natural light coming in from holes in the roof, these are air vents in the ceiling to vent any gas that may have built up.