This level set has 2 secrets in it.
A read-me file inside the game gives some hints to where the secrets can be found. It says the following:
If you want to access them, just support the UnderDucks, or you could always Bring Down the house.
The clues are the capitalised words, UnderDucks and Bring Down.
The word UnderDucks refers to this secret, which is accessed on the platform directly under the starting position of the orange ducks. Head over to the right side of the level and then bridge your way to the platform.
The words Bring Down refer to this secret. In the top-right side of the level is a hole in the cheese. The secret doorway is in here. Use your bombs to make a hole so that the doorway falls down onto the platform below so you can enter it.
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