How did Snape/Carrows find out about the secret passage to Honeydukes?

Question

In Deathly Hallows, Aberforth mentions all the passages into Hogwarts were blocked. This means that Snape and/or someone else found out about the secret passage into Honeydukes:

“There’s only one way in now,” said Aberforth. “You must know they’ve got all the old secret passageways covered at both ends, Dementors all around the boundary walls, regular patrols inside the school from what my sources tell me. The place has never been so heavily guarded.”

— Deathly Hallows, chapter 28 (The Missing Mirror)

I don't recall Snape ever finding out for certain that there was such a passage, although the scene from PoA is probably evidence enough; i.e., when he caught Harry returning from throwing the mud at Malfoy.

HP Wiki has this to say: http://harrypotter.wikia.com/wiki/Secret_passages_at_Hogwarts

Is there anything in canon to suggest that Snape/Carrows actually knew about this passageway or is it purely speculation on Snape's part?

Answer 1

I can think of a few options:

1. As PhoenixBlade9 notes, Barty Crouch Jr. had possession of the Marauder's Map for most of Goblet of Fire. Seeing it's potential use in further Death Eater activity, he tells Voldemort about the secret passageway, who then tells Snape when he becomes Headmaster.

2. Sirius and Lupin both knew about the passage, and they could have told the Order when it included Snape. In particular, I could see Lupin telling the Order and the Hogwarts staff about the passageway back when the school was heavily guarded by Aurors in Half-Blood Prince.

3. Wormtail knew about the passage as well. When Snape became Headmaster, he could have given Snape a heads-up.

4. Snape, having seen Harry at the witch both before and after his illegal Hogsmeade excursion, make the connection independently. Even though he has no idea where the entrance is, he places guards in that corridor as a precaution.

5. One of the previous headmasters (maybe even Dumbledore, given his libertine attitude towards student misbehavior) knew about the secret passage. When Snape asks the portraits for their advice on protecting the school, they are honor-bound to answer honestly.

I think 2 is most likely. It's worth noting that the Death Eaters didn't use the passage back when they were trying to figure out how to smuggle in the hit team during Book 6, even though Wormtail knew about it. That seems like a pretty good indication that the passageway was guarded back then, too.

Answer 2

My best guess would be that Barty Crouch Jr. disguised as Mad-Eye (who had the Marauder's Map and was in contact with Voldemort) would have reported the secret passageway in Goblet of Fire.

Answer 3

An answer that I'm surprised to not see here is that he didn't. Aberforth believes that all of the secret passages have been blocked except for the one in his bar. This does not mean that the passage to/from Honeydukes was specifically blocked if Aberforth did not know about that passage and had not had the opportunity to verify that it was blocked. "all the old secret passageways covered at both ends," could mean all of the passageways he knew of, instead of all of the passageways that existed.

Other than that, TenthJustice seems to have summed up the possibilities nicely.

Answer 4

Though I don't recall any specific mention of the Honeyduke's passage in this context, it is noted that Voldemort thought no-one had discovered more of Hogwart's secrets than he. It is plausible to speculate that Voldemort told Snape to block off the passage if he found it when he was at school.

Answer 5

We know Filch was at the school this time. We were also told he had extensive knowledge of the passages at the school. He could have been ordered by Snape to close or reveal the passages.