I don't remember the episode, but this happened in a flashback about Kabuto's backstory. Kabuto was on a mission and his 'mother' attacked him and Kabuto attacked her back. Then he realised that it was her and he said 'Mother' but she asked 'Who are you?' and he told her that he was Kabuto. Later Orochimaru tells him that they used gradual brainwashing on her, i.e, they showed her doctored pictures of Kabuto to make her forget how he looks.
But he even called her 'Mother' and told her that he was Kabuto, it's just impossible for her to not have recognized him.
EDIT: Some of you are saying that she was brainwashed so she didn't recognize him. But actually she knew that a person called Kabuto existed, she just didn't know that the Kabuto she saw was the real Kabuto. But since he called her mother and even told her that he was Kabuto she should have figured out it was him.
Even if he looked different, she could have guessed that he was in disguise or something, and he didn't look very different from the pictures they showed Nonou.