In Eregion Sauron posed as an emissary of the Valar, sent by them to Middle-earth ("thus anticipating the Istari") or ordered by them to remain there to give aid to the Elves.
It feels like so many problems in first two ages of Middle-Earth could be fixed if the Valar were better and more proactive communicators.
Valar (via Eönwë): Just checking up on you guys, wanted to make sure that none of the Balrog or rogue Maiar like Mairon that we failed to capture are causing any problems. We can’t intervene too much cause reasons but we might send a few emissaries your way under cover to help things along.
Celebrimbor: Oh thanks! Yeah the emissary Annatar you already sent has been super helpful!
Eönwë: ? We didn’t send anyone yet...
Celebrimbor: oh crap, guess I owe Galadriel an apology...