How To Unlock The Secret Ending
How to get the alternate ending hidden in Inside:
Find all 13 collectible round orbs with lights on them in the game and interact with them to break them all. Then right before the end of the game walk into the 14th final giant orb to disable / turn off / break the last one. Doing this will result in the black board in the background with the 13 lights to turn off, except for the 2nd light.
Which means you now have to return to below the bunker entrance in the cornfield where the 2nd orb was. Once inside, you can go to the vault door and turn the musical lever in the following order to enter the code that unlocks the round door: Up, Up, Right, Left, Right, Right, Right, Up, Up, Up, Right, Left, Left, Left.
It doesn’t seem to matter where you start entering the code (since it’s a musical sequence from the game) as long as you follow the above order it’s in. After this code unlocks the bunker’s vault door, keep walking forward to the right until you get to see a mysterious secret ending*.
*You only get to see it once, since the collectibles will be reset at the end of the game and you’ll have to collect all orbs again in order to see the secret ending another time.