"Oh," he says uncertainly, having absolutely no idea what she means. Out of her head? Out of... this location, presuming she believes the dream to be reality? Difficult to say. It's not exactly what he expected from someone of her apparent age¹. In his extremely limited experience, children usually want to make messes and watch telly. They certainly aren't concerned with being out at the wrong time.

Well, better see what she knows. "Do you know where we are?" he asks as kindly as he knows how, which is just shy of full-on patronizing, and mostly avoids being that because he is legitimately asking. Whether he means the dream or the building is up to her, and her answer will be telling.


¹ Aziraphale's visual comprehension of human age is rather shaky as it is. If asked, he might put Melanie somewhere in the 3-5 range, though she is quite apparently ten.