Andy, you want to check out the "Reconnaissance" move action. It sounds like that is what you need.
Alternatively, if the force you send is enough to "take on" any troublemakers, you want to use "Patrol" instead. In this case they will have the option to engage targets they believe they can take.
And do remember, the treaties do not take action until next season...
It wasn't a treaty as such, he said to send them, Robbie sent them. In terms of treaties I'd expect that they have to obey the local lords to a significant degree if they are on the local lord's turf, and need the Marquis' permission to do anything other than patrol and fight bandits and other marauders. Given they are there to fight bandits they would just avoid anywhere that didn't want them or any lord who would get pushy.
In practice I expect that if the Marquis doesn't have any troops himself that he 'requisitions' a few dozen of the unit as his guards and issues orders through them to the others.
I'll try and sort out a proper treaty next turn (they patrol and don't bother you, they help defend you against raiders and show respect, etc should do it) when the GMs have some time to deal with smaller npc's. Fortunately the land is thinly populated enough for the Marquis to be quite hands on and not a lot of troops are going (1 unit, say 3-4 patrols of 30-40 men - they probably go days without seeing anyone...).