You mean Bel'al, whose pet Black sisters referred to him as "Great Master Bel'al" when they captured Nynaeve & Elayne? And even if we stipulate this rather absurd point, those are all the later, secretive ones. Rahvin, Sammael & Bel'al turn up first because they're the kind who go for public rule, and leading nations, even if they use a different name. Regardless of what Semirahge and Mesaana might do several books later, we plainly see Rahvin give orders to Comar without the slightest disguise (unless his appearance throughout his run in the series is a disguise - but that's the only way he ever appears, and in his PoV he says nothing about being in disguise), and Lord Elgar (later seen among Rand's court in Caemlyn) attends him with no efforts at concealment either. The important things, at that point in the series, the Forsaken were not pulling these sorts of tricks, and Rahvin, Sammael and Bel'al were all shown to be operating in a similar fashion - they take control of a nation by inveigling themselves into the political structure. They openly and publicly weild power. By contrast, the two we see concealing their identity (both of whom are known to their minions by their names) do not hold actual power in their cover identities. Danelle holds no position of authority in the Tower, and was even eased out of Elaida's inner circle of coup-plotters. Anath might have had license to back-talk and strike Tuon, but she too had no real authority of her own or borrowed from a boss, as the sul'dam points out when she is captured. By contrast, even if their true identities were unknown, Gaebrial, Samon and Brend were generally known as important men and leaders of their nations, if not the de facto rulers. While this proves nothing about Sammael's or Bel'al's habits that might expose their identities, it still goes to show that they have more in common with the man who was accidentally overheard by a guy with no powers who wasn't even trying, than the secretive behavior you ascribe as typical of the Forsaken.

Yeah, of all that Loial tells her, that nickname is one of maybe two facts she did not know, and she even says that the nickname fits with other stuff. She was plainly NOT totally ignorant of him before that scene. And given the parallels with those two and with Rhavin, it should be obvious how a woman whose very first learned ability with the One Power was eavesdropping could have tumbled onto their secret identities.