Druidic

You know Druidic, the secret language of Druids. While learning this ancient tongue, you also unlocked the magic of communicating with animals; you always have the Speak with Animals spell prepared.

You can use Druidic to leave hidden messages. You and others who know Druidic automatically spot such a message. Others spot the message’s presence with a successful DC 15 Intelligence (Investigation) check but can’t decipher it without magic.