Secret Ending
Dirge of Cerberus has a hidden ending. The criteria to get the secret ending is by getting all three of the 'G-files'. This extra scene will play out after the 'normal' ending scene.
Chapter 8, Part 2/2
Reach the long hall after the jukebox/shop machine and climb the ladder behind the heavy machine gun and continue to the end. Successfully complete the mission 'Protect the WRO soldiers!' to unlock all the locks, making the lasergate open. Follow the passage that the lasergate was blocking, and at the end is the first G-file amongst some items.
Chapter 10
After passing the motorised cart section, you come upon a run-down road where there are some DeepGround soldiers with magical defense. Continue until you reach the rooftop and search around the corner under the stairs. The second G-file should be at the co-ordinate N-10.
Chapter 11
Close to where you encounter the airship boss, there is a room with 4 mako points. To get the final G-file, you have to stand on the plate in the centre, wait for it to lift you high up enough, and jump to the pipe. The file should be at the co-ordinate C-6, the north corner.