About Seeker and Phase arrow...
Obviously you need to know the exact square for the Phase arrow, otherwise you could not shoot it in the correct direction, because it travels in a straight line (arrows do so usually).
The Seeker arrow can turn corners and will find the target on its own (but still using your to hit bonus, it just projects the straight line of fire, you usually have, on a twisted path around corners and such).
The 'known to you' has nothing to do with location, but is the same as with many spells (i.e. Scrying), I suppose. You just have to give the arrow a target, so that the ability can work, you cannot simply shoot at any 5'-square and any target within there, but you have to specify the target itself, not its location.
The location has to correspond with your firing direction for the Phase arrow, but not for the Seeker arrow.
That's how I see it.
Bye
Thanee