"Because we were all just children. None of us knew what to do, we were all scared. And I don’t forget it, but neither does he." She always kept the Mudblood scar that Bellatrix had left behind glamoured. But she often forgot to refresh the glamor. She tended to wear long sleeves in public just in case. But at home she covered it less, both Harry and her were used to it.  However, well before she was sleeping with Draco, he often stopped by unannounced for Harry. She had only known the glamor had worn off those times, because she had noticed the deep pain in his eyes first and where he had been looking second. Since the start of this investigation (amongst other things), she had set an alarm on her wand to ensure that he never caught a glimpse of it again.   