On E!Online the article about The Killing being renewed says:
“Season three will open a year after the season two finale, which solved the Rosie Larsen case. Though Linden is no longer a detective, she’s drawn back into the game when ex-partner Holder’s search for a runaway girl leads him to discover a gruesome string of murders that has a connection to one of her previous murder investigations.”
I’m wondering if the previous investigation of Linden’s is the one that sent her into a spiral? The one she talked about while locked in the psych ward. I could be way off base, but the way they introduced her past case in S2 leads me to believe they will work on it in S3.