“Good afternoon, Harry,” said Dumbledore.
Harry stared at him. Then he remembered: “Sir! The Stone! It was Quirrell! He’s got the Stone! Sir, quick —”
“Calm yourself, dear boy, you are a little behind the times,” said Dumbledore. “Quirrell does not have the Stone.”
“Then who does? Sir, I —”
“Harry, please relax, or Madam Pomfrey will have me thrown out.”
Harry swallowed and looked around him. He realized he must be in the hospital wing. He was lying in a bed with white linen sheets, and next to him was a table piled high with what looked like half the candy shop.
“Tokens from your friends and admirers,” said Dumbledore, beaming. “What happened down in the dungeons between you and Professor Quirrell is a complete secret, so, naturally, the whole school knows. I believe your friends Misters Fred and George Weasley were responsible for trying to send you a toilet seat. No doubt they thought it would amuse you. Madam Pomfrey, however, felt it might not be very hygienic, and confiscated it.”
(Harry Potter and the Sorcerer's Stone)
In the context, ‘a complete secret’ seems to mean ‘an incident already well-known’. But I don’t find any explanation in dictionaries. What does it mean?