An Easter egg is a special hidden feature/message that a creator of a game implements.
There are very few hidden Easter eggs in the game.
List of Easter Eggs
All your money are belong to me
When robbing eight/twelve the response to rob the cashier is "All your money are belong to me". This is a reference to a popular "engrish" mistranslation in Zero Wing.
Inside of the river is a hidden area that can be accessed by swimming into the walls, this will lead into the Secret Bunker.
The secret bunker is fairly small but has some interesting things inside. There is a Jar of Souls inside and also a cork board with images of certain things linked together by red string. There also appears to be an used camera type thing hidden away and also a table with some bottles.
Removed Easter Eggs
These Easter eggs were once in the game, but can no longer be found.
Floating HouseIt is known that this floating house was actually used for a screenshot for the official game page, but the creator, Firebrand1, left it there as an Easter egg, or at least we think. It can be found near the edge of the map. Many reported that the floating house displayed different home blueprints at different times, however the house always had default painting.
The house glitch before & after: