If we really wanted to be very diligent (as I'm sure we are), we should deal with the question you made in the
Lore Corner But, I think I can give you just a direct answer here, as long as it doesn't widen itself in a broader discussion
As far as I know, the Valar sensed Ungoliant, but only when she entered directly the Blessed Realm (beyond the Pelóri).
The Valar, the Maiar and the majority of all the Elves (Fëanor included), furthermore, were celebrating a festivity at the top of Taniquetil, in the Halls of Manwë and Varda.
They had not sensed Ungoliant before due to the fact that she had always been hiding herself in the very Aman (obviously, out of the Pelóri like Eldamar and Araman), in the dark and dangerous shores of Avathar (that the Light of Valinor didn't reach).
When she revealed herself to attack the Two Trees, the Lords of the West were definitely taken by surprise, and the
Darkness of Ungoliant, described by Tolkien as the nemesis of Light itself rather than the absence of it (a sort of 'Dark Light', like what antimatter is compared to matter), had also the incredible power to temporarily darken everyone's sight (even Manwë's and Varda's) and to hide everyone imbued with it (Ungoliant and Melkor).
Yes, Ungoliant is really a terrible Maia
After she killed and absorbed the vital force of the Two Trees, she becomes titanic and it's inferred in the text that she briefly became more powerful than Melkor and the Valar themselves.
In fact, she was able to defeat the Royal Guard of the Valar and momentarily block the movements of Oromë and Tulkas!