In the rule book it mentions that the weasels lived inside the Territories before the war, so the mice could already have known about the Darkheather. The western border might have been beyond the Darkheather entrance, and it was certainly beyond the abandoned towns of Woodruff’s Grove, Ferndale, and Walnutpeck (and the warren of Darkheather tunnels noted in the southwest corner of the map, which might not have been revealed until the war’s start).
What was the relationship between the weasels and the mice before the war? Were they tentative allies? Did they simply tolerate one another’s existence? Did they trade? (Were the weasels inside the Territories simply a trading outpost?) Did the weasels make use of the scent border for protection from larger predators? The rules also say that only three mice ever entered the Darkheather and returned alive (Saxon being one of them), so it’s probably safe to assume that the relationship was a fairly suspicious one, if not downright hostile. Which would be difficult with a group living within your borders.
I’m building my own campaign in the post-war period, so I’m looking forward to everyone’s contributions to this thread.
edit: Clarity. Where’s the strikethrough feature for full edit transparency, I ask?