It wouldn't be a Metal Gear game without a little conspiracy, and fans are currently wrapping themselves in knots trying to decode a secret hidden game Kojima seems to have hidden within the code of Metal Gear Solid 5.
The game in question is called The Portopia Serial Murder Case and it's an 80s PC murder mystery that Kojima has mentioned in the past as instrumental in his decision to work in games. It was part first-person, part text, with a non-linearity and openness that meant press at the time described it as "a game without a game over", which might ring a few bells if you've seen the phrase "never be game over" in MGS 5.
So why is it important? Parts of the ROMs and game code were found in both the original release of Ground Zeros and The Phantom Pain, so fans are going crazy trying to get it up and running in case there's some sort of hidden message. The files are incomplete, leading investigators to scour the game for code. But now they're getting somewhere and part of what's currently been discovered has apparently been decoded using the hexadecimal colour values of MGS 5's lens flares.
I know. o_O
So far, while some people seem to have got part of the ROM running, they seem to have done so using a combination of the original game code in conjunction with the MGS 5 stuff (which got created slides up top, the most recent discovery). That means it seems as if people have got the original game running using the MGS 5 code but not necessarily the 'pure' code itself.
The 'Classified Intel Tape' and 'Truth' from Ground Zeroes started it all after people found code hidden in them. "The tapes are not save games and can't be combined," explains Guillian on this discussion. "The first one contains binary data (Z80 dummy code?) and the second one is some kind of BASIC program (token + ASCII parameter/value)". A Phantom Pain tape also contains a loader for the NEC PC-6001 version of The Portopia Serial Murders Case.
The conspiracy theorists are working overtime to decipher this, because if they're right some very strange stuff is going on. Most people assume it's some way to trigger a cutscene or additional content in the game. We already know MGS 5 has a secret ending which people have dug out of the code already (and they've been poking around this for a while), but people are staring to make headway now, and think it could lead to more. There are references to the original Metal Gear games with part of the code from a tape in a post-credit scene apparently translating into 'Operation Intrude N313', Snake's original mission in his first game. It's all getting very meta. This video explains what's going on with that bit although it could be spoilery (it's basically Snake putting a tape in a machine and hitting play, though, so it's unlikely to ruin much).
If it does lead anywhere, people are struggling to work out where. The Phantom Pain saves some of its data to Ground Zeroes folder, leading some to suggest that the answer is there, instead of the main game. The two loader parts of The Portopia Serial Murder Case code are slightly different between Ground Zeroes and The Phantom Pain too, and boy do people want to know why. Some are even questioning a 5GB disparity between the PC and PS4 games.
If you want to see just how far the rabbit hole goes then this guy is pulling the numbers out of everything, from The Phantom Pain and Ground Zeros and even PT, searching for a cipher.
Now that might sound crazy talk but he's convinced that he's found a sentence saying "Truth The Man Who Sold The World Is Not The Final Mission". Although at this point...
"When I came the door to/in the study was locked. So the mansion (apartment) manager, Komiya-san, came and unlocked the door for me. When I went inside the president... (as in company president) But I inadvertently looked and the key was inserted in the door from the inside. Komiya-san also seemed to notice and hurriedly checked the window and said "the room was locked so it's suicide, huh." Then I called the police." Translation by futility_jp
"Boss!! I'm your subordinate Mano Yasuhoki. Please call me Yasu. I'll explain the problem incident. The murdered [person] was Yamanaka Kouzou. He was the president of a consumer loan company."
"The first person to discover it was Kouzou's secretary, Sawaki Fumie. Kouzou didn't show up at work so this morning she came to check the situation and he was dead. Fumie spoke of the situation at the time as such."
"Komiya is also saying the same thing so it doesn't seem there's a lie in Fumie's story. So, the truth of this incident is... Boss!! I will load the main program. Please press return when the tape is prepared."