If you asked Harry how he got into Malfoy Manor, he couldn't tell you. If you asked him why he was currently under Draco's bed, he wouldn't know. Because apparently, sudden memory loss is a side effect of shock, and Harry is very shocked to find a piece of parchment entitled 'Dear Potter, if I were dating you' under Draco's bed, clearly in Malfoy's handwriting. Abandoned - sorry!