-- tells you that she's storing her luggage in Soren's office. Now you know why you want to get into Soren's office: to open Dagny's luggage and see what she's hiding there.
Go back to the Culture Center, and you should have a conversation option to tell Soren someone is stealing a sweater. (If not, go do other stuff and try again later.) Once he's gone, go into the gift shop and turn left to find the door to the office.
Despite what Nancy says, there's no time constraint here; Soren won't be back until you've found everything there is to find. So what is there to find here?
First, turn right and use the luggage key to open Dagny's suitcase. This yields a book you can't read (except for the dedication, which doesn't have anything to do with the case) and a set of flower sketches.