The empty folder

At the newspaper, we've seen a folder with a post it note attached that Casey/K.C. left for Kat at the paper in a few episodes now. The folder is seemingly blank.

One theory I haven't seen yet is that perhaps the folder still contains a message - in invisible ink. We've had it established that Evie and Colton shared messages back and forth this way. We've also seen Colton teach Jacob this method in the 90s. If Casey/K.C. is possibly Jacob's future child (or somehow otherwise related to the Landrys) as I suspect they are, perhaps Jacob would have taught them the trick to the invisible ink. I have a feeling Kat is going to spill coffee on that folder and we're going to have another reveal.