During Game of Thrones S03E06 "The Climb" the red
witch priestess Melisandre comes to the secret hideout of the Brotherhood Without Banners in order to take Gendry the blacksmith (and bastard of Robert Baratheon) with her, much to Arya's dismay.
However, given that it is such a secret hideout and Gendry ending up there was merely a coincidence and nobody seemed to know who he is at all, how did she even know where he is, let alone who he is? Did she have any kind of connection to the Brotherhood (seeing that they were also believing in the "Lord of Light") or are we to assume that all her knowledge came merely by the help of the "Lord of Light"? Or is there any other explanation that maybe didn't make it from the books to the TV show (or just not from the TV show to my attention)?