Disney Canon:
The new canon book Star Wars Battlefront: Twilight Company shows that it was possible Vader was also tracking an Imperial defector who was assisting Rebellion leadership.
(although, it could be the hubris of the character that initially led her to think he was after her, when in fact he was not in the least bit interested in her specifically)
"You found me," a voice said. "Congratulations." It was a woman's voice, in a strange, overly enunciated accent. Challis. Did you follow my shuttle to Hoth? Or did you pick up my trail later? Not that it matters, really..." She was standing a short distance in front of Vader...