Theo heard someone coming up the stairs long before Dean showed - part of the reason why stone floors were so helpful, and why he felt he could keep his back to the doorway. Either that or he was just more suicidal than merely morbid.
He spoke without turning around, unsure of who the voice belonged to but pretty secure that it wasn't a rogue DE or the like come up behind him to stab him in the back. Or a teacher. Or Snape. "No one usually is." he replied, his tone even and purely factual. "It is out of bounds."
He turned and saw that his companion was a Gryffindor, some kid that he only knew by sight. He caught sight of the sketchbook and turned back to watch the macabre pegasus fly. "Came up here to draw?" He enquired.