Did anyone know?

[Lalwendë]

There's always the possibility that those of the Noldor who returned to Beleriand knew of the Istari from the Undying Lands. I would imagine such figures would have been well known, or at least known of. There are hints that there were possibly more Istari, and characters such as Galadriel may have recognised their qualities in Gandalf and Saruman, even if she had never previously met them.

Sauron himself may have recognised them as he was on something of an equal footing, hierarchically speaking. The Istari may have travelled in the disguise of elderly men, but I don't doubt that Sauron would have been able to discern their true nature, if Cirdan could.