Sauron and Gollum

Sauron does a number of stupid things in the course of the story, but his most striking mistake is probably his attitude towards Gollum. First, he allows him to leave Mordor after torturing him in Barad Dur. Second, he sends orcs to Mirkwood to release him from the Silvan Elves. Without these two events, many things would have changed: Frodo and Sam would probably have been captured or died before they reached the borders of Mordor. Even if they would have managed to reach Mount Doom without Gollum's assistance, they would not have been able to destroy the Ring there.

Being the creator of the Ring, Sauron must have known about its powers better than anybody else. He must have known that possessing the Ring inevitably creates the desire to defend (or retain) it by any means. The Nazgul must of course be excluded here, because they are just slaves, but Gollum still has own will and personality. How could Sauron come to the idea that any other person could search the Ring for him? Do you have any idea?