With some clever writing and editing, it has been implied that Brienne is making contacts with the commoners around Winterfell, and that the commoners in turn are communicating with Sansa inside Winterfell.
I think that's all a big head-fake, and Brienne is just spinning her wheels.
The commoners are carrying messages, sure, but not for Brienne. After everything that's happened, they won't trust anyone who isn't a Northerner, and as Arya noted in S04E10, Brienne isn't a Northerner.
Nope, the commoners are carrying messages for and coordinating with... *drumroll* ...Lady Stoneheart.
Lady Stoneheart is the revenant of Catelyn Stark. A revenant is animated (literally!) by its desire for revenge. The last thing Catelyn saw was Roose Bolton kill Robb.
I think the Brotherhood Without Banners pulled Catelyn's body from the river and raised it to make Lady Stoneheart, just like in the books. But I think the show diverged from the books after that. Rather than stay in the Riverlands and hunt miscellaneous Lannisters and Freys, like in the books, I think the show-version of Lady Stoneheart is more focused in her vengeance. I think she and the Brotherhood followed Roose Bolton north and are now hidden around Winterfell, very near where Brienne and Podrick are.
I think Boltons inside Winterfell will start dying, like in the books, but they will be killed by the Brotherhood aided by the commoners rather than by the so-called "Ghost of Winterfell" in the books. I think this season's climax of the storyline will be hapless Brienne and Podrick launching their own raid on Winterfell, only to stumble into Lady Stoneheart and the Brotherhood already there doing the real work.
And I think it all might go down at Ramsay and Sansa's wedding, for some appropriately ironic payback.