Purser's Office
The Purser's Office is located in the C-Deck landing of the Grand Staircase. Frank Carlson can access it at various points in the game but after the ship strikes the iceberg, the office is closed.
Importance in the game
1) Upon leaving the Café Parisien for the first time, Frank will bump into Daisy Cashmore who asks him to look up the name of a passenger with the initials 'G.Q.C'. When he questions the Purser, the name of the passenger is revealed as 'George Quincy Clifford'.
2) Later in the game, Frank must look up the cargo manifest and leave the Rubaiyat with the Purser. When he goes to the Purser's Office, the Purser will be mumbling the name 'Thayer'. He will then explain that a telegram needs sending and Frank must offer to send it. Once he has done so, he should tell the Purser who will leave to deliver the news in person. Then Frank can have a look at the cargo manifest, where he will find details of a painting that Sasha Barbicon has stored in the cargo hold.
3) Frank will leave the Purser's Office and then go back in. When he requests the keys to the cargo hold, the Purser will tell him to go away before revealing that Isidor Straus is missing one of his cufflinks. Frank offers to find it and hands it into the Purser, who leaves his post to deliver it in person. While he's gone, Frank can take the keys from the key rack.
4) Penny will instruct Frank to check the painting in at some point in the game.