It's not unique, and Dumbledore did bring it up, actually:

"How can you not believe it? " said the Headmaster, looking completely flabbergasted. "Harry, you're a wizard! You've seen ghosts! "

"Ghosts," Harry said, his voice flat. "You mean those things like portraits, stored memories and behaviors with no awareness or life, accidentally impressed into the surrounding material by the burst of magic that accompanies the violent death of a wizard -"

[...]

I asked Hermione and she said that they were just afterimages, burned into the stone of the castle by the death of a wizard, like the silhouettes left on the walls of Hiroshima.

Compare:

For a moment it seemed like the outpouring of magic might hold, take root in the castle's stone; but then the outpouring ended and the magic faded, her body stopped moving and all motion halted as Hermione Jean Granger ceased to exist -