Area not mapped by Mapping?

[fruviad]

I don't recall having seen this before.

I've entered a new dungeon level, activated a staff of mapping, and a small portion of the level -- apparently a perma-walled -- isn't mapped by the staff.

Can some areas not be revealed with Mapping?

[backwardsEric]

Yes, one type of level has an unmappable area; they've been present since 4.2.2. See these posts, http://angband.oook.cz/forum/showthread.php?t=11322 , if you want spoilers about another feature of those levels.