I don't think Sauron even knew anything specific about the Istari mission. He knew that they were coming, that was it. In fact, when he chose his disguise as Annatar, he chose it specifically to resemble what he thought the Istari would look like. That way, if he succeeded, then he would do so in a "fair form." If he didn't succeed, then people would remember that form, and when they saw the Istari (who apparently would resemble him, according to Sauron's logic) they would immediately mistrust them, and perhaps even drive them out, thinking that they were Annatar/his followers.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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