I'm quite confident that Sauron was aware of the Istari, and acquired reports about when and whence they had appeared, which happened in response to his reappearance (c. 1000 TA).
The Istari themselves in Middle-Earth were not Maia, but Maian spirits with the limitations of a human-like bodies, and in fact, it seems that they were not too clear on their own origins, and could not and were not allowed to engage Sauron as spiritual powers, but rather to assist others in opposing Sauron, and they spent centuries simply learning or relearning what they could.
So, Sauron probably believed correctly that the Valar would not directly meddle in Middle-Earth and seek to come and get him.
And, he probably never fully understood the nature, purpose and origin of the Istari, but he probably suspected much that was correct, although Gandalf would have always been unfathomable for him.
Sauron also no doubt feared the Istari, even as he might dismiss them at times, too, knowing that the World could distract them like Radagast or corrupt them (as he himself was), and this is what befell Saruman out of despair and covetous, and potentially the Blue Wizards earlier.
Gandalf's return would have fiolled him with dread, once he learned about it, not knowing how The Ring had perhaps played a part, or some more ambitious scheme by the Valar.
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The hoes unrecked in the fields were flung, __ and fallen ladders in the long grass lay __ of the lush orchards; every tree there turned __ its tangled head and eyed them secretly, __ and the ears listened of the nodding grasses; __ though noontide glowed on land and leaf, __ their limbs were chilled.
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