When Harry noticed that Malfoy was planning something, he didn't bother informing anyone. He knew it would be useless. As always, they would do nothing. So, he didn't waste much time and took things into his own hands. Ambushing him was easy using the Marauder Map and his invisibility cloak. After disarming Malfoy and restraining him, Harry pumped the blond full of veritaserum that he nicked from Slughorn's class. After Malfoy revealed his plan, he obliviated the Slytherin back to his formative years and left him slumped in the hallway, but not before leaving his forearm exposed, revealing the wiggling Dark Mark.