I've been having a bug for awhile, occasionally I'll run into random NPC casters (un-named / generic) That have robes in their inventory, yet they don't have them equipped for whatever reason. They have their other inventory items equipped (weapon, helmet) just not the robes, It seems to happen fairly often.
Some mods I have installed:
Eotw mage ropes (which is now hidden apparently on Nexus?)
Clothing and clutter fixes
Immersive Creatures + high level enemies
Immersive Armor
Ordinator
Duel Combat
Combat Evolved
ASIS
Automatic Variants
I have created a Bashed patch and skyproc patches
Not sure if this is related?
forum.step-project.com/topic/4977-naked-npc-faldrus