What Creepin means is that you probably created your cre file from an existing one, and one of its scripts involves its behavior. So, post your cre file, please.
Regarding scripts: can you also attach your NPC's scripts, all of them? Your area script looks just fine.
Regarding walking offscreen - there's effect #293, it might be called "Enable Offscreen AI" or "Set Persistent AI" or something along these lines depending on what utility you use to check your .cre file. Do check if you accidentally had this effect attached to your .cre?
Thats the only script I have at the moment. I've only done the bare minimum for the mod right now since I'm just trying to get the hang of it at the moment. According to the guide I've been using I shouldn't require any other scripts as far as I know. This is the one dialogue file I have (which I basically just copied and pasted straight from the guide, dialogue and all).
I've been using shadowkeeper to work with the .cre file. I couldn't find anything like that in shadowkeeper, should I be using a different program?
Also, it's not precisely clear from your description does your NPC starts walking even when he is outside of the visual range of the party?
Yes, as soon as I enter the map he will start walking toward the closest companion character or PC, even if I can't see him. I've attached the .baf file to the original post.