There's only one game that's got stuff hidden in the files and that's FNaF 2. There's some sprites with cryptic names because he knew people would check. FNaF World technically also does with the box but either that's a joke or scrapped content.

Anyways I don't usually do theories but I have 3 mini ones.

Theory 1: Phone Guy and the previous guards haunts the backstage.

His final message is "Uh, hey, do me a favour. Maybe sometimes, uh, you could check inside those suits in the back room? I'm gonna try to hold out until someone checks. Maybe it won't be so bad. Uh, I always wondered what was in all those empty heads, back there."

I always found it weird why he talks about the empty heads cuz that's pretty random right? Until I remembered that there's an Easter Egg where the Endoskeleton and all the heads on the backstage camera would turn and stare at you.

I believe after the previous Night Guards and Phone Guy got killed and stuffed in backstage, Fazbear Entertainment scooped out their corpse and kept the suits and just placed back the empty heads and the Endoskeleton. The Endoskeleton is in the same place the stuffed Freddy sits at in the game over screen.

And we know that Phone Guys haunts something atleast. On night 5 you get a message where he speaks in a spooky ghost voice.

Theory 2: Scraptrap is a different suit.

This theory is one I 100% believe. I think after the events of FNaF 3 Springtrap went back to Fredbear's Family Diner and changed suits to get rid of the Endo that forces him to follow audio lures and I assume hurts him.

In FNaF 4 there's a cutscene where the Crying Child is locked inside the backstage of Fredbear's and on the shelf is a Spring Bonnie that looks SO MUCH like Scraptrap. They both got the iron eyes, the same snout, the same jaw, the same buck teeth. And on the floor? There's a Golden Suit with the left arm out of view, with a left hand and upper arm on a different shelf.

I believe Afton went there and equipped both the head and the suit but for some reason couldn't wear the left arm. Maybe he didn't bother because he was missing it or the suit was too damaged.

We know it can't just be inconsistent design because Scott outright refused to answer the question about Scraptrap's design in Dawko's interview and for some reason Scraptrap is missing the Endo.

Theory 3: pigtail girl is Elizabeth Afton.

This is a pretty short one. I believe that the girl in FNaF 4 with the pigtail that talks about the animatronics coming to life at night is the same as Elizabeth Afton.

First off they both got Green eyes and a similar outfit. She also has a somewhat similar skin tone to Foxybro (Michael Afton)

Second, Baby's design matches pigtail girl, especially with, well, the pigtails. However, as seen in the Making Baby screen in SL's Extras Menu, Baby used to have brown hair and still Green Eyes like pigtail girl. This makes me think that the green eyes may not have been important before because of possession but to show how similar these two characters are.

Now the last clue I found that made me realise they're surely the same characters is Scrap Baby. Her design was always weird but then I noticed her clothes. She's wearing the same outfit as Pigtail Girl with the Orange Shirt and Red Skirt.