Did anyone know?

[Neithan]

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.

If you are saying that the Istari were already a group in Valinor and would be recognized as a group, the my response is that they were not a previously existing group, the group was formed for the purpose. If you are saying that each Istar would be recognized individually then I would say that that is why they did not use their real names, and I doubt that anyone would recognize them in their old man form if they did not want to be recognized.