Back in the days software developers didnt announce this sort of thing and called it "easter eggs".

According to wikipedia https://en.wikipedia.org/wiki/Easter_egg_(media) :

> "In computer software and media, an Easter egg is an intentional inside joke, hidden message or image, or secret feature of a work. It is usually found in a computer program, video game, or DVD/Blu-ray Disc menu screen. The name is used to evoke the idea of a traditional Easter egg hunt.[2] The term was coined to describe a hidden message in the Atari video game Adventure that encouraged the player to find further hidden messages in later games, leading them on a 'hunt'.[3]"