I also find it hard to believe that a 5-gallon container of sewage could release enough methane to cause such a powerful explosion. But then I'm certainly no expert on methane explosions.
Well, hydrogen sulfide might do it - it stinks, and its stench is its saving grace, for it is as poisonous a cyanide, but the stench serves as a warning. In large concentrations, though, it knocks off the sense of smell, and that's what is likely to have happened. Ship's cook should be hauled off and thoroughly investigated.