Frisk the undisputed monarch of hide and seek (and massive troll) -- Gallifreya
One thing the files said about Frisk, was that if they didn't want to be found, the seeker would never find them.
One rather malevolent foster parent called the police to search for them, and the child just spontaneously turned up in the rear of a squad car. With a friend who was not so good at hiding, who had all the bruises. One of the many factors that caused Frisk to be a hot-potato style of problem child until they came into the general area of Mount Ebott.
Now that there were monsters in the world, Frisk used those skills for fun.
Well, fun for themself.
Sans had been drying out in the midday sun after gently soaking in the overnight rain. Enjoying the sky and the sight of clouds. At least until Papyrus ran screaming down the road in unadulterated panic.
After a few minutes, he went screaming back the way he had come.
Sans let it go for a few more iterations before he arranged to be in Pap's path. "Whoa, whoa. Calm down, bro. What's the problem?"
"The human is missing! I can't find them anywhere. We were playing Hide and Seek and they just... vanished. I am a horrible guardsman! I'm a failure as a monster! Toriel will be devastated!" He fell to his knees, sobbing. "I can never show my face in public again..."
Frisk, nestled comfortably in Papyrus' tattered red cape, waved a hello and signed, Tell him about 'Olly olly oxen free'.
Kid, thought Sans. You are going to be the death of me. Just... not literally... in this reality, anyway.
Sans tried to soothe his sobbing brother. "Bro. Bro. It's going to be okay."
"No it's not," sniffle, sniffle, sob. "Frisk was in my charge. They could be anywhere. Anything could happen to them."
"No seriously. It's fine. I -ah- know some magic human-summoning words."
Glare. "Why didn't you use them on patrol."
Sans shrugged. "Only found out about 'em up here. Besides, it might not have worked underground."
"And they'll work now?"
Sans grinned. "Guaranteed, bro. Got it on the best authority. All you got to do is yell, 'Olly olly oxen free'. Works like a charm."
Another glare. "Are you pranking me, brother?"
"Bro. I don't do pranks when you're crying."
Papyrus took a deep breath and shouted the words, causing Frisk to grin and slither out of their perch. Frisk circled around to where Papyrus could see them and added a smooch to his tear-soaked cheekbone.
Sans was back in his deck chair before the dramatic monologue got going.