In the episode Q Who, at the end, Guinan says that the encounter with the Borg happened before it should have. Picard says "they will be coming" in response to Guinan saying that they are now aware of The Federation's (or humanity's) existence implying this was the Borg's first contact with them as well.
However, earlier in the episode, after Q first sends the enterprise light years away, when scanning a Class M Planet Worf comments that the suspected industrialized civilization that had been there there appeared to have been ripped away from the planet identical to what happened to the outposts along the Neutral Zone in the episode The Neutral Zone.
This made me think that in both cases the perpetrators were the Borg and that the Federation and the Romulans have encountered the Borg already.....just that no one's survived to report back.
If that is the case then didn't the Borg already know about the Federation? If Guinan was expecting the Federation to encounter the Borg much later, was she not aware that the encounter had already happened and they are already seen as they are just raw material for the Borg?