I have had a corn mysteriously vanish before.. I now know that he got out through one of the little opening doors in the lid of the tank.

Are any of those ventilation holes big enough that your snake could squeeze through? Was the lid latched on both sides? Is the lid flexible enough that the snake could push up near the middle of the long sides (as far from both end latches as possible) and get out? Do you always make sure when you put the snake back in the cage that he doesn't slip out as you put the lid back on?