[Mithadan]

Boromir's analysis is sound, and reasonable. I would like to add some bricks to the path as well.

Third Age 2460, Sauron returns to Dol Guldor after retreating east in 2063 when Gandalf visits due to concerns that the Necromancer (if he was known by that name at that time) is in fact Sauron. Three years later, the White Council is formed and the Ring is found and Smeagol brings it into the caves below the Misty Mountains several years later. It is not until 2850 that Gandalf re-enters Dol Guldor and learns that its master is, in fact, Sauron and that he is gathering the Rings to himself. Saruman overrules a suggested attack at that time. In 2939, Saruman learns that Sauron is staging searches of the Gladden Fields from Dol Guldor and changes his mind. In 2941, Bilbo finds the Ring, Dol Guldor is attacked and Sauron retreats.

Sauron's retreat is tactical; it is characterized as a feint. It is, in fact, a strategic error. A few years later, he returns to Mordor and shortly thereafter declares himself. By this time, the wars of Angmar are long since over and the Nazgul have resided in Mordor for over a thousand years. While it is possible that the Nazgul assisted in the searches for the Ring in Gladden Fields, it is not stated anywhere. In fact, it is stated that the Witch King vanishes from the North in 1975.

Who searched for the Ring from Dol Guldor? Unknown. Perhaps Orcs under captains hand-picked by Sauron. He has no reason to trust his own soldiers, who are likely to pocket a bauble and bolt unless carefully watched. But after Sauron's retreat to Mordor, and temporary abandonment of Dol Guldor, there is no easy staging area from which to conduct such searches. And, years later when the Nazgul, mounted upon horses, openly cross the Anduin in search of the Shire, it is considered a significant event. It's trigger? The capture and "interview" of Gollum. Sauron knows the Ring has been found. But after the flooding of the Bruinen, the Nazgul again vanish from the North.

The point of this narrative? In the absence of the Nazgul, Sauron must rely upon lesser servants to search for the Fellowship and Hobbits at a minimum. Does every Orc need to know that the target is a "ring"? No. But given the less than trustworthy nature of the Orcs, can they be trusted to give up and report every piece of jewelry they find? Again, no. So who can he trust? The Nazgul. But the Nazgul and their flying steeds are being held on the east side of the Anduin. Yet, they are his most trusted servants. So who will watch the Orcs? A captain who respects, no, fears a Nazgul and is under his direct command, influence and oversight. Grishnakh, is this captain.

I cannot believe that crucial secrets, like the existence and possible bearer of the Ring, would be widely known among Sauron's minions. I can believe that a specific Orc captain, under the direct oversight (and intimidation) of a Nazgul and fully aware that if he steps out of line, that he will answer for it, might be told what to look for and why a Hobbit is believed to have it. And Grishnakh seems to have this knowledge ("Gollum, Gollum").