The mouth of Sauron

[Tigerlily Gamgee]

I think that's why Aragorn challenged Sauron using the Palantir in Minas Tirith... to divert Sauron's attention onto himself so that Frodo and Sam could continue to pass safely. Sauron then, I believe, put all of his focus on Gondor, rather than within his own land.
I wonder, though, if Sauron had any thoughts to what Frodo and Sam were doing in his land... or did he even know? We know that the Mouth of Sauron had Frodo and Sam's clothing, but did Sauron know about them? Well, I assume he did. Maybe Sauron thought that Aragorn was using Frodo and Sam as a diversion to take his attention away from Gondor... guess he had it backwards if that's the case.