I don't usually post my theories here BUT I THINK I'VE GOT SOMETHING!!
Okay, so:
In Undertale, there's an NPC you can come across by chance in Waterfall named "Clamgirl"
She'll tell you about her neighbor's daughter, "Suzy" (sound familiar?)
If you interacted with her, then went to the lab behind Sans & Papyrus' house and checked the drawer with the photo album-
You'll find a card! You can't actually look at it but y'know, still.
Three smiling people? That could be anyone, right? BUT WAIT! THERE'S MORE!
In Deltarune Chapter 5, during the festival, you can get ice cream from Sans. Since you get 3, he says you qualify for a punch card! (Haha Undertale ref)
He asks Susie to "draw her friends" on the card, which she does. Then, the game continues on as normal.
But in the weird route, we get more info on it!
(Kinda funny how she didn't include Noelle but whatever lol)
She draws exactly 3 people. Herself, Kris and Ralsei. Sans then puts the card away, never to be seen again.
SO HERE'S MY GAME THEORY:
Remember how in Undertale, it SPECIFICALLY said "Don't forget"? It feels like such a clear reference to Deltarune! I mean, this isn't the only reference to the semi-sequel in Undertale, far from it.
If we're going by the whole "Deltarune and Undertale Sans is the same person" theory, who knows? Maybe he took it with him and (for some reason) put it with his other photos in the Undertale world.
But hey, it's just a theory - it could mean literally nothing but i thought it was important to mention.
