I have successfully finished the game but, I am left with one unused Bahro sign from Yeesha's journal. It looks like an upright skeleton key and a snake. Does anyone know what it was for?

It's an extra clue for Noloben, where you have to use the snake symbol in one of the puzzles.

The "skeleton key" is a schematic drawing of a Bahro. It is also used in the "singing" and "friendship" drawings in other journal pages.