And more importantly, why would Siuan be spying on Nynaeve when she's way more worried about what's going on with the Salidar leadership and her own status? It's possible Siuan was spying on the same meeting that Nynaeve was spying on, but the way the chapter is worded implies that the woman was watching Nynaeve and Theodrin specifically. I don't see why Jordan would hide Siuan's identity in that sinister-sounding way. IMO, Siuan being there makes no sense thematically. Siuan Sanche, As mentions, Siuan later berates Nynaeve for spying, which she could have learned about by seeing it for herself. The two of them have several blackmail plots going on and spying on Nynaeve could have been part of that. There was a sort of on-again-off-again story about those two and their issues towards Elayne and Nynaeve for posing as Aes Sedai when they were not. But from a character standpoint it doesn't make any sense to be her - there's no good Nynaeve/Tarna connection unless she's a Forsaken, which Jordan at least knew she wasn't. Tarna also fits chronologically, having just left the meeting, and later appears to know that someone was spying. A hidden Forsaken would have likely been spying on Nynaeve and Elayne. I don't think it was really her in the window, but it's possible that Jordan meant us to think it was her, to throw suspicion onto her and keep us guessing. She was an early candidate for being Mesaana.
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People that it probably was not despite being seemingly good candidates: Most likely it was either an unused future plot hook that never went anywhere, or was thrown in to make us suspect sinister motives of other characters in the chapter. Tl dr: She's never identified and nothing significant ever appears to come from it.