[Sans has already been feeling a constant, pounding headache and nausea and a general sense of wrongness about everything when he watches Mysterio flake away with rust and distortion, ending with an explosion in case anyone might have thought that was something you survive. Sans has enough eyes in the house to know he saw Papyrus go into his workshop even if he doesn't have eyes inside, and wanting to check on Papyrus is nearly the same thing as checking on him.
A glowing emptiness tears open the space above the empty shelf, a void of light that pops as it displaces the air much like Sans's normal teleport might, before Sans is there, with too many wings that don't seem entirely committed to any one place on his body. Landing on the shelf is more of a polite nod to physics than something absolutely necessary, as Sans isn't thinking at all about how Papyrus might interpret his arrival in favor of his own concerns about making sure Papyrus isn't a melted, rusting mess on the ground somewhere.]
Papyrus? [Sans's voice is mostly the same, except it doesn't seem any more committed to coming from the right place in Sans's body than the wings are committed to attaching correctly.]
basically immediately after mysterio's death
A glowing emptiness tears open the space above the empty shelf, a void of light that pops as it displaces the air much like Sans's normal teleport might, before Sans is there, with too many wings that don't seem entirely committed to any one place on his body. Landing on the shelf is more of a polite nod to physics than something absolutely necessary, as Sans isn't thinking at all about how Papyrus might interpret his arrival in favor of his own concerns about making sure Papyrus isn't a melted, rusting mess on the ground somewhere.]
Papyrus? [Sans's voice is mostly the same, except it doesn't seem any more committed to coming from the right place in Sans's body than the wings are committed to attaching correctly.]