Missing Rune In Llevigar Plains Secret Discovery

Recently have been running through secret discoveries, and did the Spider Ruins secret discovery. In the room are runes being displayed on the walls, but one empty display appears to have been broken off or destroyed. It looks like either a Tol or Az rune was taken off, since every other rune is still in the chamber.

Is there a reason for one the runes being missing? And does it turn up anywhere else?

 

i think this is intended to show its taken some wear, but idk for sure