It is no secret that Google loves Easter Eggs, they have been discovered in the search engine with effects like tilting the list of results or showing that list as if the search was made in 1998 and there are even some interactive ones like the Pac-Man game around the Google logo. Some of their other services have hidden Easter Eggs too, like in YouTube there are animations referencing Star Trek and Star Wars. Maps also has some references to Doctor Who or might end up looking at pictures of enthusiastic employees showing some love for the company's products.
Android has had Easter Eggs since version 2.3 Gingerbread. They can be accessed by going to the About Phone section in the Settings menu and tapping repeatedly in the Android version number, in that particular version, a picture was displayed of the Android Robot along with some zombies. Honeycomb brought the Android Robot dressed up as a bee. Taking advantage of one of the most popular video memes, Nyan Cat, a similar version could be seen with the cat being substituted by the robot in Android Ice Cream Sandwich. The one in Jelly Bean brought some interactivity as you could move the floating jelly beans throughout the screen. In KitKat, users would first see a letter K and pressing it would make it rotate and then some colorful tiles would be seen making reference to previous Android versions. Finally, in Lollipop, there is a mini-game after pressing the color-changing Lollipop made with Material Design, it is a pretty difficult version of the popular game Flappy Bird.
Now Android M has been announced, and of course we expect it to have its respective Easter Egg. Some users have already installed the Developer Preview of the newest operating system and they discovered that there is indeed an easter egg present. Android M won't be the official name of the update and Google would want to make sure to keep the mystery until they launch it, so only the letter M surrounded by a circle and both elements inspired by Material Design can be seen. After tapping that graphic, you get a shruggie ¯\_(ãƒ„)_/¯ at the bottom of the screen, which may be a way of saying that they're not ready to show the final version.