Chapter IX - Secrets

The first secret you will find right after leaving the shed at the beginning of the chapter (after covering two stairs, in a box).

Another secret is hidden between two barrels in a place, where Thomas will try to lower a bridge.

The third one is in a mine tram next to the entrance of the second cave.

Next one is located in the most imperceptible place for brothers' eyes. It is placed right after you will leave the second gate (behind boxes with sign "supplies" on them, near the edge).

The last item can be picked up right after you will leave the lift. Next to the place where William will hide, near the barrel.