The Hunters did mention getting that sort of information out of Talene. signs, rituals, the way the BA communicate etc. All Talene knew (about their current plans there was not much - it's even Egwene who revealed to the hunters the BA rose Elaida... it shows how secretive Alviarin is and ought to put to rest any notion that the BA had these big meetings where they globally received orders). They couldn't make much use of that, it was way too dangerous. They were not about to go and flash BA emergency signals in front of other women... it's not like they use these signals, reveal themselves to others when they're not normally supposed to, just to say "wink, wink, we're both BA!" Just think of the circumstances they operated in and it will be obvious why they did not follow that strategy. They were not about to arrest sisters in the middle of corridors or even in private in their rooms! Before they striked at any given sister, they had to bring her to an isolated area (and in many case, deal with the warder(s) too), and when they had done that there was no point to test her with signals, they had the Oath Rod.



Egwene specifically said Verin wrote Sheriam was one of the leaders. From what I remember, Verin actually exposed the whole council or close, it's just that Egwene mentionned by name on-screen only Sheriam, Galina and Alviarin, and except for Alviarin not during reflexions of the SC but during reflexions about the specific sisters (she passed orders to catch Alviarin to the hunters before the purge... obviously not knowing that Alviarin had the hunters under watch; she referred to Sheriam as one of the leaders during a Rebel purge scenes; she thought about Galina and Alviarin as the two BA manipulating Elaida)

About Verin it's circumstancial: Verin refers in the plural to "her associates" not being happy she had to give information to the girls re: Liandrin's group but that they agreed she had to. This means was part of those discussions, either because she was a SC member, because this situation lead a summons before them, or because Mesaana got personally involved. Then there's the fact Verin reported meeting Mesaana in person twice, suggesting she was of fairly high rank (the other two sisters we know to have met Mesaana, Sheriam and Alviarin, are SC members). And of course it's suspicious Verin was able to identify SC members and even more their ranks. Whatever method they used to hide themselves worked... Talene had not even one name from the SC to offer, not even guesses.