Other realms will tend to be moderately suspicious of any strangers.
Does that mean they will arrest the noble (without an unit) even though there has not been a deceleration of closed borders?
For traders I can see how this can be a problem. You will most likely only have an open border treaty with allies and close neighbours. But traders often travel to more distant realms. Then you can hardly try to avoid every realm you did not signed an open border treaty with. Or you want to leave your realm and join another, however to get to your desired destination you will cross some realms you have not signed an open border treaty with, you get captured an return to your own capital again. Then you can start your journey all over again and hope to make it through the next time.
Aren't you just welcome to visit any realm (without an unit) as long as they have not written a declaration of closed borders?