He follows a scent, a sound, an echoing presence that has lingered in the recesses of his mind. Something that neither seeks nor avoids him, something... that is his. Was, or should be.
Sauron must find it, then. Collecting a lost pet, he calls it, though he does not remember what it was that he lost. The Orcs do not (cannot) argue when they are ushered across the plains into the southern tip of the forest, scouring the area until they find the naked hill amidst the trees. In little time his minions have dug into the barren, craggy hillside and built the stone up about them, tunnelling and making the very rock itself part of the fortress. Like Angband.
And all that time Sauron spreads the tendrils of his mind slowly northward, driving back pests while searching for that which is his, should be his. Something besides his Ring, whatever it is. Perhaps it knows what has become of his trinket. Perhaps it will even help him find it-- he is still little more than a wisp of smoke, a shadow in human form, walking beneath the eaves of the woods.
But first, the pet. While his Ring does not deign to share its location, this pet of his does, like a tug on a leash, however involuntarily. Sauron follows.