First, you need to know why you want to get back there, other than sheer nosiness. In Magnus's cabin --

Magnus's Cabin

-- you'll find a note from Dagny (on the small blue table next to the dog bed) complaining that Magnus's dog has stolen her luggage key. You can get that key by rummaging in the dog bed here.

"Dagny's Note," which you can find in the hold of the ship --

Ship's Hold

-- 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.

How do I solve the flower picture inside the footlocker lid?

Next, turn left and open the drawer to find a glove and a note that says "create volcanic winter." You'll find the mate of this glove in the ice caves.

Ice Caves

As for the note, it's a hint to help you break into Soren's tablet, which is on the left side of the counter here.

How do I get into Soren's tablet?