Secret Items are hidden items that players can interact with, found in various places. Pressing it a few times will disappear and gives you a custom contraptions, or some various loot.
Only 4 were implemented, here the list you can found:
Use the Night-vision goggles or Alien Lamp Light, you can see the bottle under the bridge surrounded by gold in Find The Statues. It’s on pig's left. This part makes the black bottle have a unique shape, and a gold mount and cap.
Also on the main screen, in the credits. Go to settings button on the menu screen, press the “i” button, and watch the credits, and at a certain point a crate will be there. Other than giving you an achievement when opened, it's no different from a regular metal crate. You can only open it once, it will not reappear after you opened it. It's always a metal crate, not any other type.