Keeping a secret is not necessarily unethical, deception is (with the same caveats as above). For instance, your employer isn't ethically obligated to tell you how much money your co-workers make. However, if they do tell you, they are ethically obligated to tell you the truth.

In this case, the author has told the employer that they are doing this work in a certain way. They are continuing to lie about it in ongoing way by intentionally including mistakes to make it look as if they did it by hand. This is unethical deception.

If the author truly believed that the employer didn't care, then keeping it a secret would not be unethical. But clearly this is not the case.