"Alright," you say wearily, "if you really must know, I used the vibrational frequency of my astral signature to set up a sympathetic resonance in any nearby spiritual bodies."
"...huh?"
"At least, I think that's what I did," you continue, massaging the bridge of your nose as another painful throb makes itself known. "I don't have a lot of experience using ki and mana combination effects yet, so I could be wrong. It could be that the pulse was etheric in nature rather than astral. Or maybe I stumbled into a practical man-based radar system. It might even be some combination of all three options."
"...what?"
"Anyway," you add, looking at the now visibly confused girl, "now that I've answered your question, I have one of my own. Why are you so insistent about hiding that you're a ninja?"
"Eh?!" Well, she threw off your attempt at baffling her with bullshit admirably quickly. Even so, you press on.
"I mean, ninjas are awesome! I know if I was one, I'd have a hard time not showing it off to everyone I knew... well, no, that's not true," you admit, thinking of Sunnydale's general population, and its "hidden" residents. "There are actually a lot of people I'd want to make sure had no idea that I even knew what the word 'ninja' meant. Not to mention the things that might try to eat me," you mutter. "Is it something like that? The classic comic book secret identity, so the bad guys don't know who you are, what you can do, or who they should take hostage to get at you?"
"...you did not just compare me to a manga character," she says flatly.
"So you admit that you are a ninja!"
"I admit nothing," she denies, in a tone that convinces nobody in this passage. From her expression, she knows it as well as you do, and she sighs. "Please don't tell anybody. I'd get in trouble."
"Let me guess: you have a teacher who believes in reinforcing lessons and disciplining breaches of conduct with some kind of low-level physical violence."
The girl remains silent, but a shift in her expression says, Oh, you have one of those, too? You nod, smiling the hard-done-by smile of the oppressed student, and a brief moment of complete understanding passes between the two of you.