I don't post here much but do appreciate all I learn here, if I may comment.
Having done AC and refrigeration work among other things we learned the principle that "vapor pressure is independant of air pressure", meaning that a leak in a refrigeration system will allow moisture in the form of vapor to enter a refrigeration system against the pressue of the gas leaking out. A friend was at a local University where the had a device that showed how water vapor was passing upstream into a room that had positive pressure as it was exhausting that pressure through the slight gaps surrounding the doors.
I, too, have heard accounts of black powder igniting after years of being loaded in a stored firearm, the one that stands out the most is the account of a bedside gun loaded, caps sealed with wax, I don't know about if the cylinders were sealed, that was in a trunk, the trunk itself had not been opened for 100+ years. when unloaded the powder did ignite.
best wishes