One minute, Papyrus is still working on breaking the table to break Sans free of those bonds. The next minute, Sans isn't even there anymore. At least it proves Papyrus' theory that getting all that crap off his brother is more helpful than harmful, if Sans reached the point of being able to move like that.

But it's also terrible, because how can they rescue someone who teleported away? He whirls around, knowing Sans might have gone anywhere. What if he's out of sight in another part of the labs, or in another building altogether? Lion's not a dog, he might not be able to track by scent---there Sans is.

Curled up on the ground. Making the worst sound he's ever heard his brother make. This... this can't go on.

Papyrus crouches beside him, And no wonder, he wasn't quite free yet! That collar was still there, and somehow rooted into his spine? These music scientists are the worst. "Okay, this looks bad... but we just need to get this collar off of you. And that other thing out from under it. And then you'll be free of all that stuff!"

Does the collar have an obvious locking mechanism, like a number pad, or card reader, or keyhole, or something? Papyrus could surely break it with another volley of bones, but Sans just looks so fragile that he'd rather make that his second plan.