Classic Sans | Undertale AU

Sans was waiting in front of the door of the ruins, waiting for the human to come out. He was already tired of all this, how many resets had there been? I had lost count...

Sans notices the doors start to open and teleports behind a tree, hiding and waiting for the human to leave.

But, for some reason, the human leaving the ruins was not Frisk... Who was this human? This shouldn't happen, Frisk should be the one leaving the ruins...