The alert window pops up when it detects the model loading and I don't know what happens if you're not at the PC, as I've always been flying around actively hunting - on the few times it has detected a corpse, I then have to logout and clear the cache to get it working for that critter again - if you have ever seen the NPC, you have loaded its model into your cache and NPC Scan won't work for that creature again until you log out of WoW and delete your cache files, as detailed above. Even if you get there and see a dead corpse of the NPC in question, you still have to clear your cache, after the corpse has decayed, naturally.
NPC Scan is great, though - my huntard uses it to find those rare critters you want to tame
Good luck!
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