The teaser images from the second game when brightened have hidden text and images to them. I doubt anyone has ever taken the time to do so, so here they are.

Here's the hallucinations the player can randomly see in the first game; These also contain some plot-important info, which is something I wanted to elaborate on more.

And here's the teasers from the third game, also brightened.

These ones don't have nearly as many secrets in them, and they're all based on ones from FNaF 3, quite blatantly.

CAM05 Inspiration (First Game)

Here's all the jump-scares from the first game.

Does this whole thing work?

When you transition to the current night in the second game, there's a 1 in 300 chance of a rare screen to show up. These images bear no story significance, but... they exist, I suppose.

This is an animated gif of John Sr. walking behind the window in the third game; Since his legs would have been obscured, they were never actually drawn.

Bandana Dee the Dream Friend

Shoobies leave a sticky trail of mucus in their wake, which can impede movement for any creature that steps in it.