Why didn't Sauron realise Frodo was in Mordor despite finding his shirt?

Question

In the Extended Edition of The Return of the King movie, in the scene at the Black Gate, the Mouth of Sauron comes out to parley with the company. Eventually, he presents Frodo's mithril shirt to Gandalf, and delivers the taunt that eventually leads Aragorn to decapitate him:

The Halfling was dear to thee, I see. Know that he suffered greatly at the hands of his host. Who would have thought that one so small could endure so much pain? And he did, Gandalf. He did.

This dialogue - and, more importantly, Sauron's finding of the mithril shirt - seems to indicate that he already knew a Hobbit had been present in Mordor. If this was the case, even at this late point, why was he not actively searching for said Hobbit? How was he unable to see the Fellowship's true plan even after he had made the very unusual discovery of a Hobbit in Mordor and had known for a long time that a Hobbit was carrying the Ring?

I'm not expecting any further exposition on this from the movies, but does Tolkien explain this any better in his writings? Or does he simply render those scenes entirely differently to the movie?

Answer 1

It's very likely that Sauron was actively searching for the Hobbits within Mordor, but the only perspective that we get is that of Frodo and Sam, so any search efforts would be happening off-screen or off-page. But, assuming that a search was happening, Sauron had one big problem. He had just recently committed most of his forces, including the Nazgul, to the war, so his available man-power was reduced. This probably helped Frodo and Same escape detection until Aragorn arrived at the gates and drew Sauron's full attention.

As far as being able to see the Fellowship's true plan, Gandalf explains that in The Two Towers.

Indeed he is in great fear, not knowing what mighty one may suddenly appear, wielding the Ring, and assailing him with war, seeking to cast him down and take his place. That we should wish to cast him down and have no one in his place is not a thought that occurs to his mind. That we should try to destroy the Ring itself has not yet entered into his darkest dream.

Even if he "knew" that Frodo was in Mordor with the ring then, according to Gandalf, Sauron would assume that Frodo would be coming to confront him and take his place, not to destroy the ring. Sauron may have just decided (rightly) that he could handle any upstarts who tried to claim the ring.

Answer 2

Sauron could not fathom that a stronger person being in the company of the ring wouldn't wrest it from the halfling. So while he knew up until Amon-Hen it was on the finger of the halfling, Aragorn showed himself to Sauron and in doing so dragged his attention away from the Morgul Vale and onto himself, Minas Tirith and the Host of the West.

Now Sauron knows all this, and he knows that this precious thing which he lost has been found again; but he does not yet know where it is, or so we hope. And therefore he is now in great doubt. For if we have found this thing, there are some among us with strength enough to wield it. That too he knows. For do I not guess rightly, Aragorn, that you have shown yourself to him in the Stone of Orthanc?’

‘I did so ere I rode from the Hornburg,’ answered Aragorn. ‘I deemed that the time was ripe, and that the Stone had come to me for just such a purpose. It was then ten days since the Ring-bearer went east from Rauros, and the Eye of Sauron, I thought, should be drawn out from his own land. Too seldom has he been challenged since he returned to his Tower.

Gandalf's plan is through distraction. He knows he has Sauron looking the wrong way, and so long as he does, he will not fear anything coming into Mordor

'His doubt will be growing, even as we speak here. His Eye is now straining towards us, blind almost to all else that is moving. So we must keep it. Therein lies all our hope. This, then, is my counsel. We have not the Ring. In wisdom or great folly it has been sent away to be destroyed, lest it destroy us. Without it we cannot by force defeat his force. But we must at all costs keep his Eye from his true peril. We cannot achieve victory by arms, but by arms we can give the Ring-bearer his only chance, frail though it be.

Answer 3

Yes, Sauron knows a halfling found and bore the Ring. But he also knows there are several halflings in play, and he has learned something of their ability to keep hidden. Given their skills to hide and avoid detection, Sauron has come to believe halflings are also being used as spies.

Weighing the lack of the Ring, plus Aragorn's challenge via the Palantir, Sauron believes he has found a spy. The Mithril shirt raises the likely status to chief spy, and therefore useful to Sauron for the taunt from the scene in question.

Answer 4

He had no reason to believe that the halfling who was caught was the Ring bearer, for the simple reason that the halfling was caught and no ring was found on him. The very idea of his enemies trying to destroy the Ring was absurd to Sauron, because he correctly knew that no one can resist the lure of power. At that point Sauron was all but certain that the Ring was in Aragorn's possession and Aragorn was bringing it right to his doorstep.