I hope this hint will help you somehow. :) You have to find a gas pipe on fire. You can do this after the firefight which is in the area with gas pipes (somewhere near the exit). So, when you find this pipe, go to the next room and turn off the object which is there. Then, go back to the pipe area and walk into the secret walkway. Climb up the ladder you will find there. On your left there will be a door, but don't forget to use a health booster, first. When you enter the door, you will be able to listen to the song (the main theme from Shogo: Mobile Armor Division). If you look at the marker board, you will understand that it is the same from the office level from Shogo: Mobile Armor Division.