After you go through the first door, when you exit the tunnel you'll be at an intersection. Take the right path and, near the Red key door, look for a faint crack in the wall. Blow it up to reveal the secret.
From the last secret, backtrack and take the left path this time. Be on the lookout for a room to your left, with a pond at the bottom. Dive into the pond and look for a hole in one of the island's walls, which is the secret.
After you blow up a wall (and jump into the hole) to make progress, take the left path (opposite to the door). Before entering the room with a Full Kevlar Armor, look for a faint crack in the wall to your left (towards the river). Blow it up and jump into it for the secret.
After entering the Red key door, notice the lava-tiled room to your right. Here, look for a brighter section of the wall to your left. Open it to reveal the secret.
In the corridor that leads to the Gold key door, near the beggining, there is a section of the wall that is brighter. Open it to reveal the secret.