When you finish a night in FNaC 1 and FNaC 1 Remastered, an interactive cutscene will play. (This is more based on Remastered, as i haven't seen the cutscenes from FNaC 1 Original in quite while.) I will now focus on the 2nd, 3rd and 4th cutscene and a FNaC 3 secret minigame.

In the 2nd cutscene, you don't see the first part, but you can hear something. At first, it sounds like some children screaming, then a conveyor belt, then what sounds like bones getting crushed. When you get to see what happened, there's a conveyor belt with what look like organs or metal parts with blood on them.

Now, let's talk about the secret FNaC 3 mingame. In that minigame, you play as Cat. Not as the animatronic Cat, but as a person in a costume. Next to you is a conveyor belt (Sound familiar?) and the Rat standing next to it. In the minigame, you grab 2 twins (You can see they're twins because their sprites are the same.) and take them to the conveyor belt. You put them on the conveyor belt, they go through a machine and, you'd except them to come out the same, but no. From the machine come Cindy and Candy. This means that the twins are now dead, crushed inside of Candy and Cindy. (You can tell that they were crushed because of the 2nd cutscene sounds.)

The 3rd cutscene isn't really that important, just the same organs with police tape, meaning that the police found the organs. But the 4th cutscene is very different. In it, the organs are gone, but at the end of the conveyor belt are Candy and Cindy, deactivated. You turn your view, then look back at them and now they are activated, with glowing white dots in their eyes and staring at you. Then, the cutscene ends.

Conclusion: 2 unknown people put on Rat & Cat's old costumes, take 2 twins, crush them inside of Candy & Cindy and they posses the animatronics now. I hope that the unknown people will be revealed in FNaC 4, as you can see somebody in a Cat suit in one of the teasers.