Egwene's been Amyrlin with the power to give orders for, what, a couple weeks? All Mesaana would have to do is make sure whatever she steals is there each time an inspection is conducted (which probably isn't often; they're in the most secure part of the White Tower inside Tar Valon itself, so OBVIOUSLY the DO himself couldn't penetrate such an impregnable wall of Aes Sedai conceit. ) Unless the last one was the night of the Seanchan raid, and given the urgency with which all the other Forsaken pursued even the rumor of angreals, it seems a safe bet Mesaana got one unless Jordan was just careless with her.

What I'd like to know is whether Mesaana was in the Tower that night, and what her experiences were. It would be excessively cheap if she wound up collared, but if she used the full measure of her strength and skill while Aes Sedai were watching she'd have some very tough questions to answer once it became general knowledge.