Man, the GMs are often plagued with \"NPC went missing! Hunt down!\" requests ;) And more often than not they\'re in some bizarre location, falling endlessly of the edge of the world. We\'ve requested a sort of invisible wall around most places you can fall off of, so if things go well such \"displacements\" will occur less and less.
Follow Nilrem\'s suggestion for now, and if it doesn\'t help or if such things happen again in the future, send a petition with the name of the NPC (something as simple as \'Rat\' or \'Rogue\' will do), its approximate default location using /pos, and how it went missing - if it just didn\'t respawn, if someone lured it away and it was never seen again, etc.
Sometimes this is quite possibly the most annoying issue.
*edit*
As for how that\'s dealt with, for curious minds - GMs have a list of NPC IDs, unique to those NPCs, and their default /pos locations. The list is gargantuan and tiresome on the eyes, but useful for just such a situation. So someone ends up /teleporting to those IDs that match the NPC name that went missing and its spawn position, one by one, until the \"right\" one is found in some place it\'s not supposed to be in. After which it is teleported back to its rightful position, and there is sunshine and rainbows :D
Except more often than not more falling NPCs are found, and it turns into a giant task.