Druidic is a language, not a code or a cipher
You speak Druidic, the secret language the druid. You can speak the language and leave hidden messages. You and others who speak this language automatically recognize such a message. Others recognize the presence of the message with a successful examination of DC15 wisdom (perception), but can not decrypt it without magic.
The only reason the news is called hidden and secret is language because people who do not know Druidic find it hard to recognize them and can not decrypt them without magic.
You must pass a check to recognize the writing
[Creatures that don’t know Druidic] Recognize the presence of the message with a successful test of DC15 wisdom (perception), which, however, can not decrypt it without magic.
Since you do not know Druidic, you must pass a check to notice the writing at all. Note that in the above description, spotting and decryption are two different things. As soon as you see it, the rune keeper's eyes enter.
The Rune Keeper's eyes are magical and allow you to read the scriptures
The description for all eldritch calls is:
In your study of the occult tradition, you have located eldritch calls, fragments of forbidden knowledge that force you to a permanent state magical Ability.
The Rune-keeper's eyes, a magical ability to be an eldritchian incantation, says:
You can read everything in writing.
So if you can discover something Druidic, you can read it with the Rune Keeper's eyes.