In mission eight and nine of the game, you're located on Nagarov's Island Base. This base has various levels, one being the security level. On this level you have the ability to lock and unlock the doors, in order to trick the robots.
Below is a map of the area:
If you unlock door number 12 (the one beside the lift to the docks) - you enter an area called Snowman World! Strange Easter Egg, but fun none-the-less.
Found another Easter Egg, or have some trivia about the game? Tell us!