The most exciting theory posits that the keyword will unlock the game’s secret ending. Mob Entertainment has hinted at a "True Ending" that requires solving a puzzle that spans all four chapters. The runes are coordinates. By tagging (screenshotting) every rune found across Chapters 1-3 and inputting them into a Chapter 4 terminal, players will unlock a battle against The Prototype.
In late October, a blurred image was leaked (or deliberately planted) on a gaming forum. The image showed a wall inside the "Prison" section of the Playtime Co. factory. Carved into the brick were three distinct symbols that match : Tag- Poppy Playtime Chapter 4-RUNE