My plan is to finally finish my re-read after I finish re-reading Harry Potter. I think I left off around book 9, and I have most of the last five in hardcover since I recently found them at a flea market.
It's possible Moghedien did sense Nynaeve but was waiting to embrace the Source so she didn't give herself away. As soon as Nynaeve attacks her she knows she's been made and immediately drops the pretense of hiding and goes into full fighting mode. That's my guess, anyway.