You know Druidic, the secret language of druids. You can speak the language and use it to leave hidden messages. You and others who know this language automatically spot such a message. Others spot the message's presence with a successful DC 15 Wisdom (Perception) check but can't decipher it without magic.
RollQuest ships the D&D 2014 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.