this scene takes place in chapter 1, when Fred, George, Lee and Harry are walking down to breakfast.
Harry, Fred, George and Lee were walking down to the great hall for breakfast. The corridors were deserted so it was more than likely everyone was already in the hall.
Suddenly Lee stopped walking. The other three boys turned to look at him in question.
"How come no one has ever seen your scars before? You shared a dorm with four other boys for four years and during the second task you took your shirt off. How come this is the first time we have seen them?"
Fred and George raised their eyebrows and turned to Harry who sighed.
"For as long as I can remember, I have been told to let no one see. Whenever he left new scars they disappeared but I could still feel them there. When I came to Hogwarts the illusion fell completely. In the morning of our first day in first year I woke up able to see every single scar. I closed my eyes and kind of just... wished they were invisible again and then suddenly... they were. Every morning the illusion would drop so I had to put it back up again. I did some research and found out it was called a glamour. I have never been able to cast one on purpose but I can apply them to hide my scars if I concentrate hard enough."
The other three boys made 'Oh' faces and they carried on down to the great hall.
George looked sideways at Harry. "You think maybe the fact that your glamour kept slipping during the night had something to do with a subconscious desire to allow someone to see? For someone to know and help." Fred, Lee and Harry looked at George in shock. "What? Did I say something wrong?"
Harry laughed. "No you didn't say anything wrong. We are just surprised you said something so insightful."
"Ah. When a smart person does something smart it is expected." George said.
"When an idiot does something smart everyone is surprised." Fred finished.
Harry and Lee laughed. "I think maybe you are right. Well. You all know now. That is something at least. Now come on. I'm hungry."