FS: Unflappably invite Pinkie into the office like a hardboiled fluttersleuth. Begin internal monologue.
Ahem. You take a good look at this bouncy broad while she wiggles her way into your office. You know her name -- everypony knows her name. You even know where she lives. The only thing you don't know is what she's doing here. And, well, how she got here, but that's probably just Pinkie being Pinkie.
"Hi Fluttershy! Ooh, it looks like you're a Fluttersleuth, I guess? I had you pegged for more of a Hysterical Dame, but that's totally cool too! Something just felt right about it, I dunno. Oh no no no, I don't mean to be rude by calling you hysterical, I like hysterical things! Hysterical is my middle name! Well, that's not true, but it should be. Hello? Fluttershy, are you even listening to me?"
The broad keeps talking, and scaring the butterflies to boot. Despite the number of words she's producing, she sure isn't saying much. Maybe you'll have to enter the conversation yourself, slip in a few questions, be assertive. That's the way to learn what you want to know.
"Wait a minute, you're doing an internal monologue, aren't you?! Ha! Can I play too? Ooh, let's see, let's see. The wizened pink Party Instigator really super-likes this office! It's pretty chaotic and the yellow inside kinda crashes with the purple outside and it smells like apples and that's just the things she does like about it. There's a Fluttersleuth in the office, which is kinda like a regular sleuth only way cuter and better with animals and there was that one time she had trouble climbing up a mountain but that was like ages ago and I'd hardly have to sing to her at all if she tried it again today!"
This broad's got the monologue thing down pat, but you're not sure if she knows what 'internal' means. Oh dear, this isn't working so well after all.
"Not that I don't want to sing to her, of course! Fluttershy's a great singer and maybe we could have a duet? Or a rap off? I don't really know what that is but I'm pretty sure it involves singing!"
"Pinkie," you say, and she stops talking in an instant, leaning forward to stare at you expectantly. "I'm, um, very happy to see you here, in the pink and all, but... what do you want?"
"Oh," she says with a happy bounce, "I'm here to help you figure out how to use your console! See," and she points to the console she climbed out from, where the passcode image has been replaced with some plain text, "it wants you to confirm that you're Fluttershy! Uh, you are Fluttershy, by the way, aren't you? Like, real nice old non-imaginary honest-to-goodness animal-loving Fluttershy?"
"Great! Then just put your hoof right there, on the top groovebutton!"