So she had not jumped up with a How did you do that?! type of question. Surprised, yes, but not scared in the least. He'd be hard pressed to alter memories if others reacted that way. Maybe she was one of the people the seal had brought here. He suspected as much, anyway. John decided to just come out with it, against his better judgment. She'd already seen what he could do. Knowing how and why wouldn't make much in a difference, probably.
So he simply paused and said to her calmly, "Infra-red scan. Martian vision, actually." He nodded. "Yes, I know that would imply that I came from Mars. Which I did. What you have just witnessed is just one of many abilities at my disposal. It comes in quite useful for my detective work."