In 4.0.1 hunters have gained control over 25 pets. This increase in pet space has led many hunters to try and catch all the spirit beasts.
Many hunters camping the spawn points of the spirit beasts use NPC Scan to alert them of the beasts presence while they are AFK or are tabbed out.
Grief Instructions
1. Set rare hunter pets to hunter ability [Call Pet]
2. Visit rare pet spawn locations and observe AFK hunters (usually hovering)
3. Call out rare pet appropriate to the spawn point
4. Dismiss Pet
5. Watch hunter go nuts looking for the rare pet
Griefing like this is very annoying. It has the potential to break the hunter's spirit. To reset the NPC Scan alert, the hunter must exit WoW and delete their creature cache file.