Because it was only ever released briefly on the PS4 and never saw a wide release, fans have never had the opportunity to properly pull apart and dig into P.T.. Thankfully, Lance McDonald is interesting in ripping apart games other than Dark Souls.
The Aussie Black Annex developer and Dark Souls / Bloodborne hacker has been digging into P.T., looking for some of the secrets and things that still remain hidden in the game world. He's released a seven minute video recently documenting his findings, providing a great insight into how Hideo Kojima and P.T. is able to build its allure.
One of his discoveries is that the game engine has the ability to hide static objects from the player's view, letting the developers hide — or reveal — certain objects when a flag is set. This allows for the creation of memorable moments, like the time when you turn around the corner and see Lisa for a brief second, only for the light to shut off and for Lisa to disappear.
As McDonald found, Lisa is actually standing there the whole time. What the game does is actually hide Lisa's character model as soon as the light shuts off. But by patching the game to prevent the change from happening, McDonald was able to keep Lisa's model viewable on screen. There's a similar moment at the very beginning of the game, and McDonald even found a prop in the game's infamous bathroom that was later scrapped (you'll see probably why in the video).
There's no jump scares, if you're worried about that. But it's fascinating all the same, almost like discovering how a magician's tricks are performed. McDonald is working on more secrets within P.T. and other games, and you can follow all of his findings over at his YouTube channel.