How to get the alternate ending hidden in Inside: Find all 13 collectible round balls with lights and interact with them to break them. Then, before the main ending of the game, interact with the last, giant 14th ball. By doing this, you will find yourself at a black board with 13 lights in the background that need to be turned off, except for the second light.
You should now return to the bunker entrance in the cornfield where the second ball was. Once inside, you can go to the vault door and use the music lever to enter the code that opens the round door: up, up, right, left, right, right, right, up, up, up, right, left, left, left ...
After that, you will open the storage door in the bunker. Continue forward to the right to see a mysterious secret ending.
Note: At the end of the game, all balls will fold, so you will have to collect them again to see the secret ending a second time.
