NPC's On Ranger/Druid Track List

[WildcardX]

Anyone know what determines if a NPC is added to the track list? What I am after here is to learn what makes a NPC untrackable?

[Angelox]

Anyone know what determines if a NPC is added to the track list? What I am after here is to learn what makes a NPC untrackable?
I don't know how, but I have an Idea on how to make an invisible (to tracking) npc with what I have now; EVENT_ENTER and maybe EVENT_EXIT - have another NPC spawn him as you get in the area maybe for a short time, then DEPOP him, Or just use EVENT_EXIT.
This would not make him completly invisible, but would spoof anyone whos trying to track.