There are some easter eggs that were not easy to find.

1.  Startup Screens

These can show up when starting the game, it crashes your game after a minute of ear piercing static

2. The purple shadow character can be seen in Camera 3, 5 and 6. The Hall, Backroom and Vent

3.  Again, the purple one can be seen in the office, the main menu and on the custom night

and Finally

4. Salvage character

He has a 1 in 1000 chance on Night 6, and any other night, a 1 in 5000