Glimp

Obscure markings left on tress, barely discernible from the natural pattern of the bark. Strangely placed rocks and sticks, as likely by chance as by design. Footprints that seems to go round in circles before continuing on again. Such strange and subtle disturbances in the natural flow of things are the hallmark of the secret language of the forestfolk. Designed to be understood by very few, misdirecting even the most accomplished cryptographers. This near-organic cypher has been used by the denizens of the deep forests for millennia, communicating everything from navigation to warnings.
Known to few, and understood by fewer still, the secret language known as 'Glimp' is used exclusively by the forestfolk of the deep forests - forested sections of the Material Plane that exist near to the Feywilds. The language is believed to have been designed to bridge the language barrier between the various creatures that call the forest home. Originally introduced by the Sprites, it quickly caught on and became a well-guarded local secret.
Very seldom being used for anything sinister, Glimp predominantly serves as a form of navigation signage, mapping out the complex maze of trees, groves and lakes. This became especially useful when sections of the forest would become warped by the leaking energies from the Feywilds. Additionally, it later became useful to demarcate areas of danger or forbidden terrain - warning of monsters, lairs and even holy ground.
Containing no specific symbols, script, or spoken language, Glimp exists entirely in the world around us. Allowing every little rock, stick or beetle to express meaning beyond their simple existence.