[Mithadan]

OK let's review what Sauron knew and what Tolkien, directly or indirectly, implies.

Sauron knew the Ring, carried by Frodo, reached Rivendell. He probably knew through spies that Frodo did not return to the Shire, though this is speculation. He knows Gandalf reached Caradhras after Gandalf conjures a fire. He knows a group passes through Moria and that the group includes Hobbits because the Orcs of Moria, whether they were free agents or not (unlikely) would pass this information on to the Mordor Orcs. He knows the Fellowship reaches Lorien. He is actively hunting for it when the Fellowship leaves and his Orcs have specific instructions to capture and not loot Hobbits. Grishnakh apparently knows about the Ring and tries to find it on Merry and Pippin. Sauron at the very least suspected that Frodo was still bearing the Ring.

Now, what does Tolkien imply? He makes much of the fact that Pippin revealed he was a Hobbit but not his name and was not questioned by Sauron about the Fellowship or the location of the Ring. Sauron was so excited he forgot to ask these questions. So excited about a Hobbit? No. He thought Saruman had found the Ring. He was sending for "it". He likely assumed that Saruman was revealing to him that he had captured the Ring a Hobbit (Frodo) by torturing him and forcing him to face the Red Eye. Or He may have believed Saruman was merely having some fun with his prisoner. I would be willing to reach and accept the concept that Sauron may have believed Saruman was still loyal, though I doubt Sauron would so lack the signature paranoia of dictators. This could explain why he sent only one Nazgul. But I am convinced that Sauron thought Saruman had the Ring.