King Regal's guards are constantly on the hunt for the Pocked Man and for Fitz. Starling warns Fitz that they're looking for him but also sings about his exploits to the caravan members, which prompts Tassin to suspect who he is. When they meet some of the King's guards along the way, it only takes a midnight conversation for Tassin to tell the guards her suspicions. Fitz is captured by them, but not before he can poison them. 

We're reading Assassin's Quest by Robin Hobb, book 3 of the Farseer Trilogy in The Realm of the Elderlings