As you proceed through the level, when you reach the Blue key door, walk around a bit to find a room which you could see from a lower level (and contained at least a Coolie). Here, on one of the rooms' wall is a small painting. It is actually fake so jump into it to get the secret.
After obtaining the last secret, exit through the painting and, right ahead of you, is a large painting on the faraway wall. This one is also fake, but jumping into it will be much more of a pain due to the small hole you have to manoeuvre in order to jump. I recommend saving since missing the jump will cause you to take damage... anyway, when you finally get beyond the painting you'll be credited the secret.
From the last secret, exit through the lower level. Near you will be a trolley. Jump on it and then onto the next section of the higher ground. There'll be a door to the left. Ignore it and proceed to the right where you'll pass a small arch-like passage, with a barred window to the right. Press the wall directly in front of the window to open the secret.
From the last secret, continue forward, down the steps, and as you proceed through another arch-like passage, one of the walls to the left can be opened to reveal the secret (actually, when you're close enough you'll see that the texture of this particular wall is somewhat strange).
Near the end of the level, right next to the Green key is a crack in the wall. Simpy blow it to reveal the secret.