Hidden bonuses: 5
Level walkthrough: Start the game by moving forward. You'll soon get to a corridor which is being patrolled by one of the guards. Press the right trigger on your joypad to zoom in the view. Move to the left and eliminate the guard with a single shot to his head. Inspect the left corridor once he's gone and you'll find the first bonus item.
You may proceed to the next corridor. Be careful, because you'll soon encounter a new guard. Keep heading forward and eventually you'll get to a room occupied by several enemy units. Once they're gone use a nearby staircase to get to an upper floor. Turn right and explore the area in front of you to find the second bonus item.
Return to the lower floor either by using the stairs again or by jumping down. Ignore the fact that the lights will go down when you enter a new room, because you won't be surprised by new guards. Take your time to find the third bonus item. Get to a new corridor and attack enemy units from a larger distance. Head forward once they've been eliminated.
The fourth bonus item can be found in the next room and it'll be located to your left. Aside from the collectible don't forget to collect ammunition as well, especially if you've already fired a lot of bullets. Find a new staircase and proceed to a lower level. Kill a guard there.
The fifth bonus item can be found to the left of the final staircase. Make sure to collect this item before moving on, because you won't be allowed to return for it. Use the stairs and you'll be rewarded with a cut-scene.