[quote="Colin"s DD talks to Harry while they are looking at the Mirror of Erised, DD says "I don't need an invisibility cloak to disappear".
He can turn invisible at a thought?[/quote]
I have always interpreted this as such...
Both harry and Ron were so preoccupied with looking into the mirror, that they just didnt' see anything else in the room, ie DD standing in a back corner, not moving around or making any noise?
Like think about it inyouf life... have you ever walked into a room, and been cso completely focused on one item in it, that you haven't seen other things in that same room???
About the Sirius/Nicholas Flamell thing! very intersting thoughts!!!