What if.......It were cleverly placed there by someone with a key to the station that is trying to get this particular crew in trouble? Lot's of theory's involved, but whats to say this crew never touched the refrigerator the 48 hours they were on?
It is conceivable that they ate at restaurants the entire time due to the stove being broken, Perhaps all should be punished for the mistakes and indiscretions of one, surveillance cameras and such.