Too many people have made interesting points to quote them all as counterpoint, so I'll just add the following:
I wonder what Xykon will want to do once they don't find Durkon & Minrah. Continue clearing dungeons as normal, on the theory that the Order can't pose a significant threat even when Team Evil is low on power? Or hunt the Order down so that they don't have to hold back some power for the end of the day? It seems unlikely he would take "that guy with the sword who's mad at him about something" that seriously, but stranger things have happened.
To amplify on why Xykon couldn't see them with True Seeing anyway, Meld into Stone is indeed transmutation... but the spell specifies Target: You. You're transmuted to fit in the stone, not vice versa.
Question: If you were Xykon/RC and suspected (not knew) this, what spell would you use?
Aside from it serving the drama, it makes sense that the odds are in favor of this being an uncleared dungeon. Iirc, MitD was marking a handful for every one actually cleared.
From a few years in Birmingham and growing up in Montgomery, I can definitely vouch for the fact that snow does exactly two things in either city (in contrast to the variety elsewhere): Land, and melt.
Sometimes there's even a significant span of time between the two.
~~~~~
(Edit: Couldn't resist adding a reply to this, I really hope that's not against a rule I haven't found yet...)
Thanks. Now I've got the mental image of Roy as a tsundere schoolgirl, "Xykon no baka!". (^_~)