I can see my neighbor's scouts?
 

Does that mean he's probably blood-hunting? Why else would they not be hiding?

Re: I can see my neighbor's scouts?
 

I you hire them they are not hidden one turn. Massed not hidden scouts also prevent some assassination spells. And they can also patrol (for a very low patrol bonus, but if you want to catch that irritating boar every point helps).

Cast a random anonymous attack spell on it. That way you can see, if they have B levels or any slaves running around they are hunting. (This is also a great way of starting a war ;) ).

Re: I can see my neighbor's scouts?
 

He might be using the scout to build a castle.

Re: I can see my neighbor's scouts?
 

They might also be holding spellcasting items and such, to bolster the PD

Re: I can see my neighbor's scouts?
 

probably it's the province where he buys them. They aren't hidden when they are recruited.

Otherwise, probably castle building.