Ok, so I've been doing more research, and I have a rough theory. That Tentacled Thing that spawned from Springtrap, specifically The Stalker, has nothing to do with this big secret. It's just a manifestation of his fears and sins.
But that's not the main thing. So judging by what Springtrap has told us about himself, as well as what Springbonnie does when in control, it would appear that her AI is far more advanced than any of the other animatronics's. While the regular animatronics, like the toys, have an AI level similar to that of, say, Jarvis, Springbonnie has an AI level closer to that of GLaDOS. Since she is the 1981 variation of Springbonnie, I'm assuming that the 1981 Fredbear was on a similar, and probably slightly higher, level.
My guess is that this level of sentience caused Fredbear to do SOMETHING. I'm not sure what, but it was bad enough that they shut down the 81 location and erased almost every trace of it. Supposedly, both suits were destroyed, but we know that Springbonnie wasn't, because William had it and used it for the murders. So what happened to the Fredbear suit? Based on what I know, here's what I think happened:
The Fredbear animatronic achieved some form of true sentience. Then, it did SOMETHING. For some reason, it wasn't destroyed. Recently, they tried to do something to it, something they tried to do to The Stalker, but it didn't work. I think this is because The Stalker is a physical manifestation of William's fear, but Fredbear wasn't. He was a self aware AI. I don't know what happened to Fredbear after, but I can guess.
I've checked everything, and only one suit in this building doesn't seem to come from anywhere - Goldy. I didn't put a soul in it, and it doesn't have an endoskeleton, yet it moves and has paranormal abilities. It's my belief that Goldy is the 1981 Fredbear. What I intend to do with this information, I don't know yet. But it's important. I have to investigate Goldy himself further.

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The puppets files
RandomHey guys. I found this in Charlottes box. It's files on me and everyone else, written in her words. Let's dive in, shall we?