And I don’t see the fact that he knew of Harry’s encounter with Lucius as needing any particular explanation; it happened in public, with at least one person who might have spoken to Dumbledore about it standing right there.
I just went and reread “38. The Cardinal Sin”. The only strange remark that Madam Longbottom heard is:
“Of course...” said Lucius slowly. “I do feel the fool now. This whole time you were just pretending to have no idea what we were talking about.”
It’s not obvious, to put it mildly, to get from this to “writing notes about a long mysterious conversaion”.
EDIT: Hmm. Or maybe it is obvious to Dumbledore...
I just went and reread “38. The Cardinal Sin”. The only strange remark that Madam Longbottom heard is:
“Of course...” said Lucius slowly. “I do feel the fool now. This whole time you were just pretending to have no idea what we were talking about.”
It’s not obvious, to put it mildly, to get from this to “writing notes about a long mysterious conversaion”.
EDIT: Hmm. Or maybe it is obvious to Dumbledore...
I think it kind of is, actually.