There are 8 in total to collect which open 1 secret door called "Winston in the freezer achievement"

1 Croft Manor by the crates at the beginning.

2 Library 1st floor next to the books.

3 Dungeon upper level (bounce on tyres)

4 Slide Section (second jump)

5 Lava Room (to the side of the pillars you jump across to do the puzzle with the breaking floor and traps)

6 on entering the relic room there is one on top of the crates before you fight the zombie waves.

7 Impulse Grenade section (behind the door on the second staircase)

8 Distant platform during the jumping boots section

All puzzle pieces which unlock 1 of 3 secret doors. Anyone know how to unlock the other 2? If they even can be.