Arnie, was the context "betrays?" If so, I think it is used wrong. Yet, could it mean that only the fact that she is wearing a headscarf is hiding the fact that she is suffering from breast cancer? In other words, she looks pale, exhausted and ill, but the headscarf makes her look perkier? I doubt it! I think he/she just used the word wrong!
Yes, the only way it could be used correctly would be if the scarf were to have hidden the fact that she had cancer. However, that is very unlikely since many people with cancer treatments lose their hair and then wear scarves to hide their bald heads. So it is likely that the woman looked vibrant and healthy, but the scarf betrayed the fact that she is suffering from breast cancer.
I don't have the paper now, but I seem to remember that the article started by explaining that the picture was of KM in walking Paris with her boyfriend and commented how well she looked. The sentence I quoted came next. The author obviously meant betrays but I'm surprised a sub-editor (US: copy editor) didn't spot the mistake. It wasn't the Grauniad, after all!
Build a man a fire and he's warm for a day. Set a man on fire and he's warm for the rest of his life.