Aziraphale takes a moment to study her, considering her answer. She seems very certain of herself, very aware of her surroundings, fabricated though they may be. He knows humans are capable of creating a great deal in their dreams, but this seems far too specific. From what he's learned of ROMAC from Crowley and general hearsay, it's not at all unlikely that they do have this sort of 'high-security' area, but to what purpose? And why is this little girl dreaming she is there?

Even more confounding is her little theory. She delivers it with a seriousness he's never seen in a child, and he is inclined to match her gravitas, if only to play along.

"What do you mean?" he says, cautious, curious, only a little indulgent. "What might you 'do' to me?"