When you kill the scout while its camp still exist, that scout will eventually respawn after some time (how much? never was able to find out really...). But I think the algorythm for scout respawn is at the closest tile from the camp. It that said camp had spawned some troops, and the IA enclosed them, maybe the closest tile would have been that valley?? It is still strange, since the cities are quite old and the skirmiser is rather close to the player troops (so shouldn't have spawned that long ago). Maybe the camp was on Swidnica location, and quickly destroyed???
Maybe there's a camp in that other mountain gap next to it? Still an unrevealed tile there, and this location would be the "closest" location to that camp