From the start of the level, go west. After you walk down the stairs, you will find yourself in a room which is an entrance to a maze. At the beginning, you can go only to the south and west, but when you push the switch in the western corridor, a northern corridor opens. In the maze, there are 3 small niches, each one behind a fake wall with a gargoyle face and a stream of blood. Each one contains a sergeant, and each one is in different corridor: southern, eastern and northern. These three niches count as one secret.
From the entrance to the maze, choose the northern path and turn right twice (opposite to the blue skull key). Find a niche similar to those from secret #1 to get an energy cell pack.