The poster is the secret sex scene for her, and you can get the poster by repeatedly going in and out of cam 3 again and again until the poster changes to golden freddy's face.
I tried it later though, and it doesn't seem to work for me anymore.
I believe they are referring to the easter egg golden Freddy that appears if you click on Freddy's boobs too many times on the poster in your office, not the poster that randomly shows up in the closet and spawns golden Freddy.
