Based on the intermittent Handplates period when Sans and Papyrus knew how to write English, but couldn't speak it and signed instead. This leads to a lot of pandemonium.
Feel free to let me know if you have any more ideas!
Sans' POV :
I don't know how long it's been since the last time I left the house, but today (y/n) left Snowdin.
My thoughts were interrupted by a fizz noise, as if (y/n) was running her finger on the door. It had a sequence...
~one long, pause, one long, one short, pause, one long, pause, two short, pause, one long, three short, pause, one long, pause, one long, pause, one long, end~