Mystery Door

Description
The mystery door (also known as the secret door) is located in the top area of the lobby.

The door is a grey-ish blue color and has a black question mark on it.

Launch Event
There is nothing behind the mystery door, standing next to it would only reveal empty space around the lobby.

Holiday Event
Reactl0n and mPhase are behind the mystery door, sitting in front of a fireplace and drinking hot chocolate.

After Holiday Event
There is nothing behind the mystery door, standing next to it would only reveal empty space around the lobby.

Before Simulator Event
If you stand next to the door and position your camera on top of it, you can see text behind it:

"The mystery door will unlock in a future update!"

Simulator Event
Reactl0n and mPhase are behind the mystery door, throwing cream pies at each other.

Unlocked during Easter Event
Past Reactl0n and mPhase, you see a path leading up to The Scrambler. There are also boxes next to the path with items/decorations from past events, like a decorative Festive Bat and Snowball Cannon from the Winter Event and a disco floor and disco ball from the Launch Event. You can also see the trap posters that are normally in the lobby and have been temporarily replaced with "simulator game" thumbnails.

After Easter Event
There is nothing behind the mystery door, standing next to it would only reveal empty space around the lobby.

Trivia

 * During the Easter Event, the door would close after you claim The Scrambler. However, if you collect any of the eggs in the maps or lobby again, you can trigger the cutscene and unlock the door again.