Is it considered undutiful to one's mother to do something she forbade if she doesn't object when it's done in front of her, or if she is unaware that it is the very thing she forbade?

Obedience to parents is obligatory in everything that benefits them and does not harm the child. The mother's silence does not indicate her consent unless there is an accompanying indication. Otherwise, disobeying her command is considered disobedience (to parents), and her ignorance of what you have done does not justify your action.