During the "Titan's Realm" mission, you will reach an cavern area with lots of enemies and a yellow booster platform at the back. Walk up the stairs to your right and follow the wall along the second floor to the end. There is a hidden room behind a crack in the wall that you can jump up to reach. Although it looks like it is too small to get through the crack, you can enter the wall to find a skeleton with a party hat on and a "Happy Birthday!" balloon and cake next to him.