Professor Dumbledore isn’t just magic, he’s an immensely useful man to know. After all, he has the habit of showing up just in the knick of time to save Harry and his friends. (Until, tragically, he’s no longer about.)
As Reddit user ScottTheAmazing puts it:
He is eerily aware of a lot that is going on, especially when it had to do with Harry. Just take the first movie. The first real interaction Dumbledore has with Harry is when he's looking at the mirror of Erised. He says "Back again" as if was fully aware of each time Harry had been there. At the end of the movie Dumbledore talks to Harry about specific details of his adventure…later at the feast in the great hall, Dumbledore also gives accolades to Hermione for being calm, Ron for playing a great chess game, and of course Harry for showing bravery.
How could Dumbledore have known about any of these things?
The answer, apparently, is right under (or on top of) your nose.
This all becomes explainable if he's actually seeing what Harry sees. In fact one of the best pieces of evidence is found with Dumbledores brother Aberforth. Harry acquires a shard of a mirror (which is glass) and he can see Aberforth. The Dumblefore family has clear evidence of using magical glass to see what’s going on somewhere else.
Yes, apparently Dumbledore can see everything Harry can see, whether that’s in the classroom or…anywhere else.
What’s more, apparently this surveillance has run in the Potter family.
I really like the idea that Harry's glasses are actually his fathers glasses. Spectacles would but unusual for a kid to have, but if they were passed down to him it would make more sense. And if you look, of course you see that the spectacles they both wear look exactly the same. Perhaps when James Potter was in the order of the Phoenix Dumbledore enchanted them so that he could be aware of what was going on. This also would explain how Dumbledore knew some intimate details of when Harry was killed.
Well that’s a terrifying, if annoyingly convincing theory. However, one commenter suggested an alternative explanation to Dumbledore’s seeming omniscience.
My theory was always that Dumbledore knew so much because was using legilimency on Harry. The book referenced a few times that Dumbledore had blue eyes that seemed to look through Harry, or something like that….During those times, Harry actually notices Dumbledore subtlety performing legilimency on Harry. This wandless legilimency works in part because Dumbledore is powerful enough to do it without a wand, but also because Harry is not good at legilimency. Harry's complete trust in Dumbledore might also make him easier to read.
So, either total surveillance or mind reading. Still think Dumbledore is such a good guy?