Sans is sitting on his windowsill seat in his room. He is staring out his window watching the snow fall.
Then he is in a room similar to his own. When he was younger he might have had this room. Then the door opens and he hides in the closet. Through the crack in the closet doors he watches two people enter the room and a very familiar voice rings out.
"It's time to go to bed sans."
The small sans on this person's shoulder holds on tighter and starts to protest.
'but dad I'm not tired.'
"Will you at least try to sleep, for me?"
The small sans nods and is set down on his bed.
While this has happened Sans has stiffened and his eye lights have fizzled out. As soon as the small sans has been set down Sans teleports to the hallway of his "childhood" home.
'why is this happening?!?'
Sans crouches on the floor with his hands behind his head. After a few moments he hears the door open again and he teleports to the forest behind his house.
He starts to visibly shake and leans up against the tree. He teleports deeper into the forest and collapses in the snow by the base of a tree.
POV Change
Gaster knew that there was a soul in the closet similar to his child. So he put sans to bed and left the room. He noticed that the soul jumped from the closet to the hallway. And then from the hallway to the tree closest to his house.
He put on a couple of layers and went out the back door, into the woods. He followed the soul into the woods and as he got closer he could feel the turmoil in this soul, so similar to his sons.
He gets into visual range of this soul and he can tell why it felt so similar to his sons. It was because in a way, this soul was his son.