Exactly the kind of experience she flatly refuses to entertain in cases until forced. She has been holding out on him, if not - he assumes - on purpose. This can't be something a person simply forgets, but maybe she'd found ways to excuse the experience. Weird trips, stress about the murders, all kinds of strange little pieces of her life coming together to make her hallucinate. It's a thin explanation to cling to, but it sounds reasonable on the face of it. Scully loves reasonable, and as a kid with no prior experience with the supernatural, she must have needed to find a way to explain to herself what had happened.

"What happened?" He's not convinced that seeing a flash of a dead girl would be enough to burn the experience into Scully's memory; anyone could imagine that by accident, thinking of a death when everyone else is thinking of it, too. Something more must lie beneath her explanation.