My Theory.
Something in the Hp POA film shows something that happens in book 6 or 7
hmm.Remember that some wizards can become animangi
which is a good thing to be, so most wizards powerful enough would be one yeah.....
Well Think of dumbledore
Think of how he can see through the invisibility cloak
think of how he has the sparkly eyes
think of how he looks old and elegant yet is still agile.
Take a look at this
Cool theory, but I don't think Dumbledore is an Animagus.
He's blind (no I am NOT crazy). His eyes are magical, like Moody's but, Moody's eye is so because he was an Auror (give's a creepy effect on his rivals), that's why he can see through an invisibilty cloak.
Another reason: never noticed why Dumbledore has such a good hearing despite his age, blind people have it too.
And the reason that he kinda knows of everything that happens in the castle, i ve got 2 assumptions: it's rather the Chocolate Frog card with his face on it or rather it are the paintings that are all over the castle ( remember the talking in OotP in his room with the paintings).
Ladyslytherin, he has magical eyes every time he sees a magical thing in or about harry, the eye receives this en thus "twinkles" as the eye sees the magic. Just bare with me a Little.
We all know that Harry is very powerful, but doesn't know how to use it.
But actually he's greatest power is his believe. If he does believe he can do something, than he will do it. And almost every time he's with Dumbledore, the Professor gives him couple of instructions or rather believes. Harry trusts the professor with his own live, thus he starts to believe to and then the magic in him begins to work althus creating the "twinkle" in Dumbledore 's eyes, that are just only picking the magical signature of Harry's evolving powers started by his believe.
Hope I have made myself clear and answered your question.