You could be right. Corianin may have found a loop hole around her oath not to rat out the Black Ajah by hiding the notes very well instead of destroying them.
Corianin was the last known Dreamer in the Tower before Egwene, she lived 473 years before Egwene came to the Tower. She studied ter-angreal, among them sleepweaver ter-angreals.
We discover that Verin is a darkfriend at the end of the book. Verin gave Egwene a sleepweaver ter-angreal that she...