The Hidden Village

Sans, Papyrus, and Gaster went missing a few years after Gaster got out of the void. A search party was sent out for them but after a few weeks nothing was found. They were declared dead. This affected everyone, but Grillby was affected the most.
Sans was Grillbys best friend and Grillby could trust Sans with all his heart. Once Sans was declared dead Grillby was devastated, Grillby took a few weeks off work to calm down, Fuku would've helped Grillby but she was at a faraway collage and couldn't make it home, and she moved a month before the skeletons disappeared. So Grillby was all alone.
The night before Grillby was supposed to go back to work he was in his room crying, telling himself that the skeletons weren't dead, and that Sans would be back in a few weeks. He couldn't hear his window opening and someone climbing in and sitting on the windowsill through his sobs.