I got my Sans plush (and Papyrus plush too cuz I can’t separate the bros) and celebrated my joy with a bunch of Sans drawings. Then I realized my sketches were becoming darker…
More reasons why I think Sans has reason to test Frisk for ‘outside influence’ (yours):
Frisk already canonically has one spirit of a person able to influence their actions, and you’re another one
You absolutely are a character in the game. You’re part of it. Chara talks to you if you No Mercy everyone. You make Frisk do (almost) everything. You nudge them into actions, which Chara (possibly) communicates to you through Frisk’s eyes (item/monster descriptions, evident since the game text is an unreliable narrator in any game route)
Sans knows SOMETHING is wrong, something capable of reworking the flow of time to its will. [our reports showed a massive anomaly in the timespace continuum. timelines jumping left and right, stopping and starting… until suddenly, everything ends. heh heh heh… that’s your fault isn’t it?] It’s manipulating time itself, nothing is off the table here, and a human suddenly showing up underground after that is suspicious.
Sans is unbelievably observant, to the point of noticing changes in the timeline even though he doesn’t retain his memory. He can read Frisk’s EXP and LV somehow, he can count how many times he’s murdered Frisk by their pre-battle demeanour, his actions change if you reload enough times.
Sans literally follows you through the game because he’s suspicious and watches your actions and how you react to stuff. Every interaction he has with Frisk and the odd things he does are probably a kind of litmus test for Frisk as a person, but by extension, whatever weird oddity exists alongside Frisk
He hid the door to his lab in a place the player can’t see from their perspective. He does lots of things that only make sense from our (the player’s) perspective. Appearing between trees in the forest before meeting us, the conveniently shaped lamp, MAKING us watch him scratch his butt (thanks), his constant ‘turning to the camera’, staring at us, or winking at us
There are certain things he doesn’t say out loud. Certain things come across only as text. “Y o u ‘ d b e d e a d w h e r e y o u s t a n d” is unvoiced. He communicates only with the player and maybe Frisk hears nothing. It makes sense with his jump in tone – for Frisk it was a vague, hanging threat, and to us, it was an explicit warning that he is NOT fucking around even though his world is a toy in your hands.
So, he has enough understanding to use his telescope prank not just for funsies, but since he knows something about the player’s perspective, he’d know the significance IF Frisk were to use the telescope, look away from him, then immediately confront him about the blotch on their eye that they can’t see.
It’s also consistent with his general purpose as a meta-character who half-understands what’s going on. He knows about you reloading a save, he basically reads your character sheet back to you if you push him, and he sort of understands that something exists in Frisk that can toy with the world. So, to protect himself, he tries to amuse us so that we have a good time playing the game and we want him and the other characters to survive. Unlike everyone else staying put like most RPG NPCs, he’ll teleport ahead of you and do his little antics to be entertaining, not unlike his hobby as a restaurant comedian. Basically, he knows you’re a Protagonist and decides to show you a good time. He wants you to let everybody live. He might be too anxious to JUST be a lazy doof, so he goes kind of overboard, and the pranks are probably fun for him too. Or on the other hand, maybe he knows what happened to the other kids and wants to cheer Frisk up in case they’re terrified about ending up the same way.
Plus, Determination isn’t really Frisk’s ability. It’s ours. We’re the ones who load, save, and close the game when we want. Frisk inherited this power when we began existing within them, and presumably, it then leaves Frisk normal again when we stop playing, able to live their lives without us, because everything is safe and nothing resets if we aren’t there.
Basically, Sans acts like a character who knows he’s seeing The Main Character walk around and that some weird consciousness (us) is doing things in their world and refuses to take this lying down, so he taunts us, pranks us, and maybe shames us for being so powerful in a programmed world. He tries to defy rules in general, like gravity or common sense. He’ll put 29 hotdogs on Frisk’s head and then randomly stop once they’re off the screen.
There’s an implicit relationship between Sans and the Player because, like with the timeline manipulation, he sort of knows the player’s there, he acts certain ways for their benefit, and he tries to gauge what they want. He might be the only character besides Chara who can directly talk to you, and at the very least, he pointedly aims parts of some conversations your way.
tl;dr Sans is halfway to the understanding that he’s in a game and wants the player to like him and have fun so they’ll let everyone in the game be happy. He also messes around with you to defy being in the role of an NPC as much as he possibly can, to stop you being complacent in your omnipotence