Isn't Hunters that Prethoryn mention are in fact Unbidden?
Unlikely. The Prethoryn mention that they follow them everywhere they go with inter-galactic space as their only safe hiding place. But the Prethoryn use some kind of biological Warp not Jump drives, which makes it unlikely that the Unbidden have any recognition that they exist. The Unbidden also are interdimensional invaders and should have no particular reason why intergalactic space would be any safer from them than anywhere else if there's enough intelligent creatures there to be worth purging.