I have a need to give NPC's AI packages that take them out of circulation for a while. E.g. a merchant that travels out of Cyrodiil to adjacent provinces. The AI system doesn't provide for "nowhere" as a location, so what other options can I use to simulate it?

One thought is a interior cell that belong to the "abroad" faction, and a teleport door somewhere in Cyrodill that leads there. Level 100 lock, belongs to that faction. So they can go there, but the player can't. Needs multiple doors, one for each route "out" but could share holding cell. Door need not be a door, but could be a rock or other static.

Another is to have a script disable the NPC's when they reach an appropriate point near the border, and a timer re-enables them when they're due back. Needs markers at the borders to trigger script on a GetDistance test, so it's probably as much work as the doors above. Plus a script running to monitor it.

Any other ideas?

Do you need them to actually get that close to the border? I usually just place a marker somewhere and set up a travel package to travel to that marker. Once they are out of the player's sight, I disable them. But if you want the player to be able to run into them on their way to the other province, then yeah, you'll need to place a marker near the border. I'd still disable them when they reach the marker. If they aren't essential, you'll have to make sure they aren't dead when you want to reference them again.

I usually have a master quest script that controls all of this kind of stuff. It would re-enable them when appropriate. Or just have a separate quest for this. I don't think it would be too bad.

I was using the term border here to mean "exit point, as I have a town outside the borders and at least one of the NPC's (the merchant) lives there, so his "border" is even further out.

I'm currently testing with a rock as a door to a holding cell, to see how the AI works out. Your point about disabling and essential is a good reason for leaving that option for later.