Cp01 Contains 5 secrets.

Secret1 : In the starting area, if you leap into the water, you'll see that the one wall is covered in moss. You can actually pass through this, leading to small area where you can surface. Surface and explore the ledges.
Contains : Diving suit and a life essence

Secret2 : Before the area containing the skull key there is a bridge. If you turn to your right you'll see a piece of wall with two wooden beams on either side. Push this wall and it will open to reveal a chamber with some items.
Contains : Napalm launcher, 2 gasoline cans and a life seed

Secret3 : Once you've entered the chamber beyond the skull key door, there will be a small passage roughly in the center of this chamber. There are shelves on the sides of this passage. At the end of the passage is a switch. Pull the switch and a door nearby will open.
Contains : 2 boxes of flares and 2 boxes of shotgun shells

Secret4 : In the areas surrounding the dagger key door there's a grandfather clock. Push it to expose a small chamber.
Contains : Doctor's bag

Secret5 : In the room next to the one with the grandfather clock there are some shelves with skulls on. Push it to lift the shelves and find your prize.
Contains : Super armour

CP02 Contains 3 secrets (out of 4. The fourth one doesn't add to the secret count.)

Secret1 : If you enter the opera house and go up the flight of stairs on your right, there will be a large, green tapestry with a unicorn on it. You can simply walk through it and enter a secret chamber.
Contains : A box of flares, a life seed and body armour

Secret2 : In the dressing room there is a cupboard. Open the cupboard and push the wall behind the clothes. It will lift to reveal a secret passage. Follow this passage to find your reward.
Contains : A box of shotgun shells and a full drum of bullets

Secret3 : In the room with the organ, where you acquire the dagger key, play the organ. The bookshelves on your left will revolve.
Contains : Nothing, but you can use this secret passage to access other areas

Secret4? : After passing through the moon key door, enter the first door you find on your right. There are two bookshelves in the corner. Press the one on the left to reveal a secret space.
Contains : Doctor's bag and limited invisibility

CP03 Contains 4 secrets (out of 3!)

Secret1 : Press the painting to the right of the eye key door.
Contains : Life seed

Secret2 : Press the bookcase you find just before the fire key door.
Contains : DEATH MASK

Secret3 : In the room with the fire key there are some bookshelves. Press one and it will lower to reveal a secret space.
Contains : Guns akimbo

Secret4 : In the eye key room there's a bookcase behind a table. Press it and it will lower to reveal a secret chamber.
Contains : Life seed

CP04 Contains 7 secrets (out of 6!)

Secret1 : Before you enter the first building in this level, use the bench on your right to leap onto the higher ground.
Contains : Tommy gun and 2 boxes of flares

Secret2 : As you enter the first building and collect the body armour, look for a ledge above the staircase on your right. You can actually leap up there. You'll find a switch that opens a door leading outside where you originally collected the body armour.
Contains : Napalm launcher, a box of TNT and 2 life essences

Secret3 : There's a room where you pull a switch to open a door elsewhere. There's a voodoo doll and a gasoline can on top of a box in this room. Just before you enter said room, push the shelf with barrels on. It will open to reveal a small chamber.
Contains : On lower difficulties a life seed and on higher difficulties a life essence

Secret4 : After passing through the moon key door you access an outside location. There is a house built into the main structure that has a chimney releasing smoke. Search along the wall of the house for a small, dark object hidden behind a tree. Press this object and a nearby door opens.
Contains : Tesla cannon

Secret5 : When you enter the church-like area with the cross with flames behind it, look on the side of the altar. There's a skull. Push the skull and the shape of the cross behind the altar will change, allowing you to pass through in a secret chamber.
Contains : Life seed and a life leech

Secret6 : After falling down the well and surfacing, climb the rocks and look at the waterfall. Your prize is quite visible, contrary to its nature.
Contains : Limited invisibility

Secret7 : If you follow the river and leap onto the rock surface with all the spray cans, there will be a spider web hiding an area containing a shelf with a wine bottle on. Press the shelf to lower it and a passage will open containing an interesting elevator. Activate the elevator to ascend to your prize.
Contains : Life seed and 2 tesla charges

CP05 Contains 3 secrets (out of 1!)

Secret1 : There are two small lifeboats hanging near the back of the steamboat. Leap into the left one.
Contains : Life seed

Secret2 : There are two small lifeboats hanging near the back of the steamboat. Leap into the right one.
Contains : Super armour

Secret3 : In the engine room, leap through the flames into the furnace. It's a secret chamber.
Contains : Life seed and fire armour

CP06 Contains 6 secrets and the path to the secret level!

Secret1 : Look at the left wall (the hedges) on your way to the chamber with the skull key. One of the pieces of the wall is differently coloured. Press it to expose a secret spot.
Contains : Life seed

Secret2 : Just before the passage leading to the skull key chamber, check the grave in the corner. In it will be a switch. Pull it to open the nearby door.
Contains : Guns akimbo

Secret3 : When you enter the room with the red bricks and wooden stairs, after going up the first set stairs, turn left and push the wall.
Contains : Doctor's bag

Secret4 : In the first room with the fireplace, where all the body vaults are, push the first one on your right when you open the door. It will open.
Contains : Super armour

Secret5 : In the room where you acquire the eye key, look to your far right, after staring at the eye key. There are three boxes of TNT. Now go to the opposite end of these wooden beams and push the wall. It will open.
Contains : 2 basic armours and a doctor's bag

Secret6 : Just before the stairways leading to the end of the level, jump down the fireplace you find.
Contains : Exit to the secret level!

CP07 Contains 3 secrets.

Secret1 : After you first enter the mountain, keep an eye on your left. In the first cavern will be a platform you can leap onto. Climb the rock surface and go through the nearby passage to find a prize near a perilous edge.
Contains : Life seed

Secret2 : After going over the first broken bridge and crossing the narrow ledge, press the wall to your immediate right, past the pillar. It will open.
Contains : Tesla cannon

Secret3 : After using the forklift to jump to the steel door you opened earlier, go around the corner, past the control panels and push the walls one will open to reveal a secret chamber.
Contains : Life leech

CP08 Apparently contains 2 secrets.

I have found several locations in this level that should be secrets, but none have any effect towards the secret count. I am here to tell you about secrets, not would be, or should be secrets. For this, I apologise.

CP09 Doesn't specify how many secrets it has, but I will point out some.

Secret1 : Just before you pass through the second door in this level, push the wall to your left, behind the skull. It will open.
Contains : Life seed, sawed-off shotgun, a box of shotgun shells and spirit armour

Secret2 : Before you pass through the third door in this mission (in the water-filled area) leap into the water on your left and swim for a short while, then surface.
Contains : Beast vision, a napalm launcher and two gasoline cans

Secret3 : In the room with the fireplace, go into the fire, it contains a surprise.
Contains : Tommy gun

Secret4 : In the large room with all the pillars, just after you get the limited invisibility, look to the walls for one that has long, narrow, vertical slits. Push this wall to reveal an area with a life seed and a button press the button and you'll find that you're on a lift.
Contains : 3 tesla charges

Secret5 : In the hallway with all the suits of armour (knights) in, look to the wall on your right. Something on the wall between the first and second knight doesn't appear quite right. It's actually a button. Press it and a suit of armour on the opposite side of the chamber will lower to reveal a secret chamber.
Contains : Life seed

CP10 Contains 0 secrets.

Not very secretive for a secret level.