The Hoop and Arrow
Reene Harfor informs Elayne that Milam Harnder visits the inn every three to four days, after which the innkeeper send a pigeon north. Three of the Aes Sedai, all Brown Ajah, staying at The Silver Swan also visit the inn and meet privately with the innkeeper. Proving the innkeeper and Master Harnder are Brown eyes-and-ears.
(Reference: Crossroads of Twilight, Chapter 14)