Dream Theater hid a (decidedly less wholesome) message in Morse code in one of their songs, and it took fans a while to find it even after the band revealed tha

Dream Theater hid a (decidedly less wholesome) message in Morse code in one of their songs, and it took fans a while to find it even after the band revealed that something was hidden in the song (third bullet point from the bottom here https://en.wikipedia.org/wiki/Train_of_Thought_(Dream_Theater_album)#Songs https://en.wikipedia.org/wiki/Train_of_Thought_(Dream_Theate...). I imagine that hiding these sorts of messages in songs would be fairly easy, but making them possible to decipher for a passive listener without being too obvious seems a lot tougher.