Even if Xykon and/or Redcloak suspect the MitD of marking unexplored doors, there's always the question whether or not they could do anything against it. My money is on the "MitD is a young Tarrasque" theory, and in that case, Team Evil would need a Wish spell to rid themselves of the MitD. And while Xykon would certainly be powerful enough to cast Wish, there is no indication at all he has it on his spell list, especially since Wish is pretty much narrative-breaking like True Resurrection.