So....
http://undertale.wikia.com/wiki/Sans#Personality
"Sans may have a scientific background. Evidence includes the quantum physics book, workshop, his relationship to Alphys,[6] affinity for science,[7] and his timeline research.[8]"
And we know he has "Gaster Blasters", the name from which comes from the filename for their image/sprite.
Sans has a scientific background, Gaster was a head scientist... In Delta Rune, Sans claims to have recently moved into town. Furthermore, the locked door south of town isn't mentioned by anyone. Could this lack of mentioning be because it's a new development?
Said door has slowed down Gaster sounds coming from it....
Sans is also very interested in how Kris behaves. Consider some of his DR dialogue tree:
Check out (almost) all of his dialogue in a 5 minute video:
https://www.youtube.com/watch?v=UQceGfnSP34
Dialogue quotes & REALLY wordy theorizing:
Sans: "hey, look who's walkin' around. how are ya', kid?"
Notably, he says the first sentence looking normally, but he asks his question with his eyes to the side & a grin, like he's trying to hide his amusement at some big joke that's coming.
And then Kris can reply.
Kris: "Great to see you again"
Sans (same expression as with his question): "yeah, it's real nice, isn't it?" Then, winking, he continues "especially considering I've never met you before." Normal faced: "the name's sans. sans the skeleton. i'm new in town."
OR
"Who the hell are you?"
Sans, looking up & to the right side instead of left: "hmm... y'know, good question." Winking: "i'm sans. your friendly neighbourhood skeleton."
In either case, he finally asks "what's up?" with his eyes to the left.
We can also get a phone number from him. Supposedly his, which is soon dismissed as not his, & just gives us a one time call to the hotline for idiot babies. Yet if Kris answers anything but 4 to it, & tell Sans he should be friends with Kris....
Sans, eyes to the left: "ah, sorry. i don't hang out with idiot babies. " winking: "they compete with my sociological niche."
Lastly, there's the bird cage that the soul gets put into at the end of the chapter, which has been known to have suffered some crashes.
So how could this all tie together? Well, it seems like Sans was expecting us to answer a certain way.
"Look who's walking around. What's up, kid?" is a very familiar-sounding way for a stranger to greet a kid.
And he almost seems amused if Kris responds like Kris knows him, Sans being happy to inform him they don't. And maybe it's just being a joker to such a rough question of "who the hell are you?" but Sans answer seems... surprised.
"hmm... y'know, good question. i'm sans. your friendly neighbourhood skeleton."
And maybe his phone line prank was prepared for anyone to be given to, but if he does indeed say he doesn't hang out with idiot babies UNLESS Kris selects 4,
then that would mean Sans KNEW what Kris answered the phone line with because he knows what Kris selected no matter where he makes that 1 call.
And then the line doesn't pick up again after 1 answer.
Not only is it strange for a hot line to only take 1 call from a certain number -& the menu even claims it isn't Sans's own number after calling it- but Sans in Undertale doesn't like the player in Undertale's Genocide route,
one of the stereotypes he associates with them being people who feel they must do anything because they can; Completionists.
Wouldn't it make sense someone who's anti-completionist set up a hot line that you can only use once? And he seems to have monitored it, & given how he greets Kris, he probably intended to give it to Kris once they started chatting.
The cell phone Kris uses even makes Gaster noises in the Dark World, someone Sans almost certainly worked with, given his scientific background & use of GASTER BLASTERS.
THE POINT:
What if Sans moved into town to work with Gaster? Or live closer to his work with Gaster? His dialogue heavily suggest Sans works at the grocery store, yet he doesn't have it open this evening, & he certainly doesn't seem willing to open it for Kris.
Could Sans's grocery store job be a cover-up job?
And what if he knows about the birdcage that traps the SOUL? If that birdcage has seen a lot of crashes, what if Sans or Gaster have observed it, or even moved the cage around before? What if they know about the SOUL?!
Heck, we even know that Sans wants us to hang out with his brother Papyrus tomorrow, & he makes it clear he won't be there. But why wouldn't he be at his own house? Even if he's working at the grocery store, his home is literally next door.
(Which would make it easy to claim he was at the grocery store if he returns from elsewhere to visit Kris & Papyrus.)
TL;DR -
I think Sans knows about the SOUL in Kris, or at least, suspects it, & he & Gaster are working together.