Duergars in the mines

I'm wondering if this is a bug. I see no reason why these duergars should be able to see duo. <447hp 675m 498mv 13993tnl> n A Dark Junction in the Mines [Exits: north south] A dark-skinned dwarf stalks the dark tunnel. You step back into three dimensions. A duergar scout steps out of the shadows. A duergar scout's backstab === OBLITERATES === you! That really did HURT! A duergar scout is in perfect health. <265hp 755m 504mv 13993tnl> A duergar raider's slice MASSACRES you! A duergar raider's crush MASSACRES you! A duergar scout is in perfect health. <126hp 755m 504mv 13993tnl> fle A duergar scout parries your stab. A duergar raider's slice DISMEMBERS you! You sure are BLEEDING! You dodge a duergar raider's crush. A duergar raider's slice DISMEMBERS you! You sure are BLEEDING! A duergar scout's crush misses you. A duergar scout's pierce DISMEMBERS you! You have been KILLED!!