When Sans stepped out Papyrus had assumed he just needed a break from the excitement of the party. But the longer Sans was gone, the more worried Papyrus got. Eventually he decided to call his big brother. Only to hear his phone in the cushions of Lady Toriel's couch. This admittedly wasn't uncommon. He often 'lost his phone to the couch' when he didn't want Papyrus to know where he was. That was mainly when he went to Grillby's, so Papyrus wasn't panicked yet. Still, he hoped Sans would return soon, Toriel was his best friend after all, his pun buddy.

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Papyrus bid his goodbyes and went straight to Grillby's. He'd waited a half an hour longer before panic set in. He truly hated when Sans did this, Papyrus was constantly worrying for nothing, but he couldn’t help it. Sans never told him anything even though he knew something was wrong. He'd seen the nightmares, he'd seen the pain and sadness in Sans sockets. Papyrus used have to pick a drunk Sans up from Grillby's. Papyrus had tried to confront Sans about his problems, but it Sans evaded the topic... like he always did..

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Sans wasn't as Grillby's like Papyrus hoped. He wasn't at the house either. Sans was just gone. Papyrus didn't know what to think about this development. Sans was never not where Papyrus could find him. So he called Undyne. "Hey punk! Why didja leave early?"
"Sans Is Gone! I-I Don't Know What To Do!" Undyne huffed on the other end of the phone. "He's not at Grillby's?" Papyrus shook his head. "That Was The First Place I Checked." She grumbled. "And he's not at home?"
"That's The Second Place I Checked! Undyne, I Know My Brother's Hang Out Locations. Even The Ones I'm Not Supposed To Know About." Papyrus replied. She grumbled louder. "Ok, Ok. Maybe he was homesick or something. He probably went to the Underground. If he's not back in 24 hours, call the police then call me." He nodded, feeling a little relieved. The Underground was a reasonable location for him to go to. He liked to hide in Waterfall. Or maybe he was at the Underground Entrance, waiting for the stars to come out. Sans loved the stars. Didn't mean Papyrus still wasn't worried. "Alright, Undyne. Thank You.." He could imagine that shark like grin of hers. "Of course! Bye Pap!"
"Good Bye."

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"And could you give us a description of your brother, Mr. Papyrus?" Sans never came back. Papyrus was scared. Undyne was gathering their friends and coming here to help. "S-Sans Is About 4'11", Thick Boned, With A Smile, He's Always Smiling.." The officer gave a reassuring smile. "Don't worry, we will find him in no time. What was he last wearing?" Papyrus took little reassurance in the officer's words. "He Wears A Plain White T Shirt With The Same Ketchup Stained Hoodie And The Same Black Shorts With The White Stripes And Faded Pink Slippers.." Papyrus had always yelled at him for not washing his clothes. Now he was somewhat glad. They would find him easier. "I see, thank you, Mr. Papyrus. I assure you, we will handle this. Your brother will be found." Papyrus gave them a picture of Sans before they left. Not long after, Undyne came with everyone. "Ok! Don't worry Pap! While the humans search the city, we will search the Underground! He's sure to be found in no time! Then I can beat the crap outta him for scaring you like this!" So they started searching. When the murders increased, Papyrus feared the worst. But he stayed hoped for the best outcome. Sans would come back.