After being refused by Feanor, Morgoth went to southern Valinor to seek out Ungoliant and the two of them destroyed the Two Trees, murdered Finwe and plundered Feanor's Vault and stole his Silmarils and fled.
My point is: how was Ungoliant in Valinor already without being noticed by the Valar, Ungoliant was a huge spider formed evil spirit and dwelling in the lands of the Valar. Was she already in Valinor before the Valar resided there?
This is answered in the passage of the Silmarillion introducing Ungoliant:
...she fled to the south, escaping the assaults of the Valar and the hunters of Orome, for their vigilance had ever been to the north, and the south was long unheeded.
In other words, the Valar weren't looking in that direction.
Ungoliant was in Middle Earth from the beginning and may well have settled before the Valar. She lived in the deepest darkness below some mountains beyond the Southern border if Valinor in an area called Avathar (I will add the quote later when I have the book to hand). It is possible this darkness combined with her own dark webs shielded herself from the Valar. Furthermore she appears to be as powerful as the Valar as she defeated Morgoth in a fight. The Valar might have known she was there but not wanted to fight such a powerful enemy especially with Morgoth already at large fighting two powerfull evils might have been beyond them so they focused on Morgoth.
