Well, the imperials wouldn't raze any village if they could avoid it. They would, however, do their best to capture anyone who might reveal their existence to others. Rather than slaughter or raze anything, they might just temporarily imprison the village. But here's the thought:
Maybe Bor expected someone from his village to meet him, but that doesn't happen because they're prevented from leaving? That in turn might make him curious and/or cautious, right?

We'd rather not a random (nameless) NPC revealing the imperial invasion. A PC, on the other hand? not as easy to prevent. We'd guess, once he figures out what is happening, that Bor might reach the city around the same time as the imperial army. Remember, they'll have begun marching out of the tunnels the moment the darkness rose, for at that point, they won't be visible. We'd guess the tunnel exits to be not far outside the darkness. Short enough to have a quick march to get there before the darkness stops again, but far enough away not to arouse suspicion. Maybe 4-6 km or so away from the walls?

It would make sense for the Imperials to send outriders in all directions away from the tunnels, to ensure that there's no one warning anyone about their existence. If everyone assumes the Imperials came over the mountains, which is the more logical route after all, they'll how the Empire supplies and reinforces its forces.