3:00pm - Meet with 'you know who' in the empty science lab 
Sunset bit her lip as a drop of sweat made it's way down her face with agonizing slowness. She had mere seconds before her window of opportunity would close on her, and she couldn't let that happen. With a deep breath she gave the dial one last turn and gasped in relief as the locker made a satisfying click. Giving a quick glance up and down the empty halls and confirming she was alone, she popped open Trixie's locker and began rummaging through a large pile of stage magician props.
After a few moments, she felt her fingers brush against the curled wire spine of a notebook, and grinned in triumph as she pulled her treasure free. Predictably, the cover read 'The Great and Powerful Day Planner!' in large, glitter adorned letters, which caused Sunset to shake her head with a chuckle, even as she admitted to herself that it was kinda cute.
Refocusing on the task at hand, Sunset opened the little book and thumbed her way to Today, scanning through Trixie's day and finding exactly what she was hoping for scheduled just after classes would be dismissed for the day.
3:00pm - Meet with 'you know who' in the empty science lab
Sunset grinned like a timberwolf that had just stumbled on a Hearth's Warming Dinner...wait, that didn't make much sense, maybe a regular wolf and a Crasmus Dinner...is that what they called it here...something like that...
Sunset's deliberation was jarringly interrupted by the sound of the bell ringing, and it took all her willpower not to scream and toss the planner in fright. Instead, she hastily shoved it back into the locker and slammed the door, before moving down the hallway while trying to look casual, hoping she could put some distance between herself and the scene of the crime before slipping into the crowd of students moving between classes. Luckily her classmates remained as inattentive as ever.
Alright Trixie, let's see what you're hiding, and if it's what I think it is, Princess Twi is gonna get a real kick out of who her doppelganger has been sneaking around with. It was another act of will that allowed Sunset to not slip back into her previous grin as she made her way towards her next class.
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Sunset huffed and grunted as she tried to army crawl her way through the air duct that she had managed to sneak into. "Alright let's be honest here, I think this is officially going too far..." Sunset grumbled to herself, although at this point it'd be harder to give up then it would be to just follow through on her ridiculous stealth mission.
With a little bit of a struggle, she extracted her phone from her pocket and managed to pull it up in front of her, quickly navigating to the picture she had saved of an old blueprint of the school. "Ok, if I'm reading this right, it's a left turn and then just a few more feet to get to a vent that looks down into the science lab." she whispered, putting her phone back into a pocket and wriggling further down the duct.
In a matter of minutes, she had reached her target, and settled into a more comfortable position as she examined her new perch. Hmmm, I mean I can see down into the room, sure, but only like, a few square feet around the door. I'd even say I would have had more luck looking through the keyhole at the door...I've been watching too many spy movies.
It was at that moment she heard the sound of pacing footsteps, and realized somebody must already be in the room. The rhythm of it even sounded familiar, making Sunset think it really might be Twilight standing just out of view. As quietly as possible, she began trying to contort herself in a way that would allow her to peek through the vent towards the far corner of the room, but there was just no way to do it without making too much noise. Resisting the urge to groan in frustration, she looked at her watch, which she could just barely read in the semi-darkness, confirming that Trixie should be arriving any second, if she wasn't the one that was already here.
Right on cue, the door beneath her opened and she saw Trixie walk into the room...and then promptly out of her line of sight.
Ponyfeathers! Sunset mentally cursed, resolving to just listen, it shouldn't be too hard to recognize Twilight's voice after all.
"Hey you." Trixie cooed. "I've been looking forward to this all day..."
Sunset waited for the other person to speak, straining her ears, but had to restrain a yelp when instead of a verbal affirmation, she instead heard a series of wet, sucking noises.
Damn Twilight, I didn't peg you for the sloppy makeouts type, let's hope you're better at it than a certain someone I know. Sunset thought with a good natured eye roll, thinking of the only human she had kissed before.
After a few increasingly uncomfortable minutes, the noises finally stopped and Sunset resumed actively listening in.
"Hmmm that was nice." Trixie said breathily, before returning to a more normal tone. "As much as I enjoy these little meetings, I wish we could just tell everyone we're together."
Jackpot! Maybe I'll finally find out why she's been keeping this from us, it's not like we'd tease her for dating Trixie...too much...
"I know babe, I'm sorry." Came a voice Sunset instantly recognized, though it wasn't Twilight's.
FLASH!?
"It's just..." Flash Sentry continued, sounding a bit flustered, "...ever since the whole thing with Sunset, and then the weirdness with two Twilight's, I'm just kind of nervous that if we tell everyone, it'll all fall apart."
Sunset couldn't help but cringe a little. Really didn't need that indirect guilt trip right now, but I guess I kind of deserve it.
Trixie audibly scoffed, though Sunset could still hear the sappy smile in her voice. "That's a bit silly and naive of you, but I understand, take all the time you need."
Aww, well isn't that sweet, those two make a cuter couple than I would have expected.
"Now in the mean time, I know just how to make you feel better." Trixie continued, her voice suddenly growing husky.
Sunset heard the sound of a zipper.
BAIL, BAIL, SWEET CELESTIA, I GOTTA GET OUTTA HERE!!!
It was at that moment that it dawned on Sunset that she didn't have enough room to turn around in the vent, and ahead of her was only a dead end. Her eyes shot wide with panic as she realized she only had one option.
"WAIT, WAIT, STOP! I'm stuck up here and I really, reaaaaaaally don't want to play witness to this." She whined down through the vent as the room below her went chillingly silent.
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A bit of help through the vent, and then one extremely awkward conversation later, Sunset made her way down a hallway with a determined glint in her eye and Rarity trailing at her side.
She had decided that, given how spectacularly the stealth approach had blown up in her face, her best course of action was to just confront her next suspect directly. Unfortunately, given who it was, she was going to need some backup, and Rarity had been the natural choice, given her romantic inclinations and awareness of the investigation in general.
The pair reached a door at the end of the hallway, which was clearly marked with a sign reading 'Emergency Exit - Alarm Will Sound'. Not hesitating for even a moment, Sunset shoved the door open, and headed through, followed by Rarity and a distinct lack of alarms. It was common knowledge, at least among the students, that someone had taped over the alarm sensor for this particular door.
Stepping out into the fresh air of a well hidden internal courtyard, Sunset glanced to her right, finding her target sitting on an upturned milk crate with a half finished cigarette between her lips. "Hey Adagio."
Adagio Dazzle took a puff from the cigarette before her lips split into a sinister, smoking grin. "Sunny Bunny, Miss Priss." She purred, giving Sunset, then Rarity a nod.
The pair both resisted the urge to offer a very dramatic eye roll. They were familiar at this point with Adagio giving everyone a cute but condescending nickname.
"Come for a smoke?" Adagio asked, languidly offering a little paper box towards the two of them. "I didn't figure either of you as the naughty type, but I'm more than willing to share." she continued with the tone of someone who wasn't talking about cigarettes anymore.
Rarity seemed to fluster a bit, but Sunset was visibly unaffected. "Actually I came to ask you a question."
Adagio slowly stood from her seat, giving an exaggerated stretch that mostly just pressed her ample chest forward, then strolled towards Sunset, not stopping until they were maybe an inch apart. "Anything for you Sunny Bunny." she said in a sultry voice that had no place on school grounds.
Rarity's cheeks grew redder, while Sunset didn't even blink. "Cool, uh this might sound a bit weird but, you're not secretly dating and/or doing Twilight Sparkle are you?" Sunset asked, then quickly added, "the non-pony one that is...guess that's kinda obvious?"
Adagio's grin grew wider. "I can't say I've had the pleasure unfortunately."
Sunset gave her a questioning look. "I'm not saying I don't trust you, but you know I gotta ask, is that the truth?"
Adagio leaned forward and whispered directly into Sunset's ear. "I'd never lie to you Sunny Bunny." She finished the statement with a little growl, then she backed off and flashed both of them with her best bedroom eyes. "Though I will say I do have my eye on a few of you Rainbooms, now that we've cleared up all our bad blood...mostly. And dare I say a 'pony one' is right at the top of my list." She ended her sentence with a wink.
Rarity looked ready to faint, while Sunset just put a finger to her chin in contemplation. "Huh, well good luck with that, I'm not sure Princess Twi has quite forgiven you guys as much as we have."
Adagio's seductive demeanor finally dropped in favor of confusion. "Th-that's not really what I me..."
"Well anyways," Sunset interrupted, "That's all I really needed to know, thanks Adagio, catch you later!"
Both Rarity and Adagio watched Sunset leave the courtyard with matching looks of disbelief. "She...she really is completely oblivious isn't she? I had heard the rumors but I guess I didn't really believe it." Adagio mumbled, staring at the empty space Sunset had previously occupied while taking a fresh drag from her cigarette.
"Oh you have no idea." Rarity agreed, trying to discreetly fan herself now that it seemed Adagio had turned off the heat. Of course it wasn't to last.
"So what about you Miss Priss." Adagio purred, turning her full attention towards Rarity as she tossed away the cigarette butt. "I'd be lying if I said you weren't pretty cute yourself, and you seem infinitely better at flirting." she added, batting her long lashes.
"I-I-I..." Rarity stuttered, trying to summon the words to turn the Siren down as her face began to redden again. "I-I...oh who am I kidding. Let me give you my number." she groaned as she fished around in her purse, finally giving in to the girl's wicked charms. "This wasn't supposed to happen until college." She muttered, even as a girlish giggle made it's way through her complaints.