It wouldn't be Metal Gear Solid without some extra Easter eggs. Kojima's playful sense of humour is given free reign in the Déjà vu mission, which hides great rewards for fans of the series.
Meet Hideo Kojima
The very important person in the VIP mission is none other than series director Hideo Kojima. His reaction to Snake changes depending on your mission rating. Get an S rank and he'll give you some high praise. Worth it. Once rescued, be sure to linger on the mission select screen with Koj in the background to witness another mischievious moment.
Play as Solid Snake and Grey Fox
Collect all of the XOF patches and you'll unlock the Déjà vu mission, a nostalgia trip for series fans. Complete the mission with all flashbacks unlocked and you'll get a Metal Gear Solid quiz. Beat that and you'll get the classic Playstation skin for Solid Snake. Complete it on hard mode and you'll get Grey Fox. Replay the mission as these guys and enjoy the added nostalgia of a snowy Camp Omega, patrolled by the Genome guards of MGS1.
Once you've unlocked the MGS1 Solid Snake skin head back into the Déjà vu mission and return to the place you'd normally find Chico and Paz. You'll find they've been replaced by the president of ARMS-Tech and the DARPA chief, both dying of the FoxDie disease. It's best not to spoil how this one plays out.
Find Johnny's dad
Also in the Déjà vu mission, check out the bathroom stalls near the prisoners and you'll hear the harrowing sounds of a soldier enduring infinite evacuation. Listen to his moans and gurgles as long as you like, they never seem to end. This is a reference to the colonically challenged Johnny from MGS1 and MGS4, though given the time difference between those games and Ground Zeroes, this must surely be Johnny's dad.
Hear a message from Psycho Mantis
The Déjà vu mission is bursting with neat secrets. This one requires that you find the logos of prior Metal Gear Solid games scattered throughout the base. Erase them all with the torch on the AM MRS-4 rifle for a tiny message from Psycho Mantis.