It was the subtle changes that gave it away. Nothing was ever said or done to outright give it away, but the trained eye could spot the changes.
It was in the look the headmaster gave Draco Malfoy when the boy acted out for attention.
It was in the quick, short smiles the two exchanged when no one else was around.
It was in the fact that Draco's misbehavior began to follow a schedule, and in the fact that each breach of the school rules sent him straight to the headmaster rather than allowing the teachers to deal with it themselves.
It was in the careful way the boy walked the morning after those trips to the headmaster's office, and the smirk on his face.
It was in the tears he shed on the old man's grave.
And, if you looked closely, if you took the time to read between the lines...
It was in the note he left, the day he died himself.