An important hint:
“Obliviation cannot be detected by any known means, but only a Professor could have cast that spell upon a student without alarm from the Hogwarts wards.”
If only Zabini had spoken more with his older cousin, he might have remembered that five years ago, Defense was taught by Professor Hat-and-Cloak.
More seriously, if Dumbledore thought to make sure no one could bring the same Dementor back the next day, he probably wouldn’t neglect to revoke, say, Slughorn’s permissions.
An important hint: “Obliviation cannot be detected by any known means, but only a Professor could have cast that spell upon a student without alarm from the Hogwarts wards.”
This means no Lucius, no Sirius, no Lupin, etc.
Very Interesting. Are we certain he doesn’t just mean that Hogwarts only detects student’s spells? Probably not.
Do you think individuals who were professors in the past but are no longer might not be detected?
If only Zabini had spoken more with his older cousin, he might have remembered that five years ago, Defense was taught by Professor Hat-and-Cloak.
More seriously, if Dumbledore thought to make sure no one could bring the same Dementor back the next day, he probably wouldn’t neglect to revoke, say, Slughorn’s permissions.