Isaiah adds a personal note and plea to the specific prophecy. He begins with his testimony that he does indeed follow the Lord. Isaiah uses a contrasting image to show his devotion. He uses the image of a hidden God and Isaiah seeking him.
Of course God is not hiding himself from Judah, but rather that Judah does not find him because they will not look. Isaiah, on the other hand, does look.
Isaiah's evidence that he has looked to God is his own mission, and the signs from God represented in his children. This is a clear reference to the dual "child" prophecies, the first for Immanuel and the second for Maher-shalal-hash-baz.