Nothin` new-
60 years ago we kids used to heat bricks red-hot in our coal furnace in the wintertime, put them by tongs into the cutter`s steel foot boxes to keep our feet warm. When we returned after a 2 hour jaunt through the snow later, we would take out the bricks by tongs and throw them into a large pot of water which would immediately start to boil for a long time.
Duhhhhh!! Nuclear Snow!
Hawaiians also use red-hot stones to cook the pig in the ground for hours. ``Hey! Look!! No power!! But we got lechon!! [Phils]
Duhhhh -
Big Kahuna Nuclear Power.
No, that sort of thing has been easily ruled out. A passive heat source would have cooled off early into the self-powered portion of the test. The more relevant possibility is that the 'mystery box' that was not opened was large enough to have some kind of mundane chemical fuel source that kept reacting at an even rate, producing the heat. But it would have to be something that gives off no fumes and results in the unit weighing the same before and after (that was checked and reported by the observers).