[Zigûr]

In "Notes on Motives in the Silmarillion" in the "Myths Transformed" section of Morgoth's Ring Professor Tolkien observes the following regarding Sauron:

"If he thought about the Istari, especially Saruman and Gandalf, he imagined them as emissaries from the Valar, seeking to establish their lost power again and 'colonize' Middle-earth, as a mere effort of defeated imperialists (without knowledge or sanction of Eru). His cynicism, which (sincerely) regarded the motives of Manwë as precisely the same as his own, seemed fully justified in Saruman. Gandalf he did not understand. But certainly he had already become evil, and therefore stupid, enough to imagine that his different behaviour was due simply to weaker intelligence and lack of firm masterful purpose. He was only a rather cleverer Radagast - cleverer, because it was more profitable (more productive of power) to become absorbed in the study of people than of animals."

I would argue from this that Sauron must have been aware of the Wizards for some time, but was not especially threatened by them, any more than he was by the Lords of the Eldar, at least, and was at least in some regard ignorant of their nature and purpose. I daresay this was part of the plan - not only did their humble shape make them readier counsellors rather than leaders for Men, but eluded the full attention of Sauron, who surely underestimated them, and Gandalf in particular.