And also in the previous episode Blood Fever, they found the Borg corpse in the Sakari ruins.Yes, the first hint they get of increasing Borg presence is when they find the planet of de-assimilated Borg in the episode Unity, and the dormant cube floating in space nearby. Little did they know that they were flying into the heart of Borg territory - and no one on the planet thought to tell them what was ahead.
Has it ever seemed like if the Borg were such a big threat, why wasn't the whole DQ assimilated by then? Were the Borg not already around during the height of the Vadwaar civlization and that was centuries before.