Right at the start of the level, after you exit the cell, look to the left to spot a crack in the wall. Blow it up to reveal the secret.
After climbing the stair, proceed past the Silver key door, take the right path at the intersection, and when you reach some windows, move to the right window and look for a crack in one of the walls (if you look from the outside the window will frame it perfectly). Blow it up to reveal the secret.
From the last secret, walk along the outside path, looking for a crack in one of the big trees. Blow it up and jump inside for the secret.
After opening the Gold key door, when you reach the pond area, climb the left building (the one with a Riot Gun) and look for a crack in the wall. Once again, blow it up to reveal the secret.
After destroying the fixed machine gun, investigate each of the lighter-colored tree sections: one of them will hide a very small path behind it. Going here will count as a secret.