A poster is hidden in one of the outer walls of the tower, accesible only by dropping from a window (thankfully, there's no fall damage).
The door locked by a red stamp (mentioned above) leads to a hidden cinema where a short Serious Sam film is being projected.
note Strangely enough, at some points he starts going on strangely philosophical and sometimes outright nihilistic rants, but it might just be him screwing with the player's head for kicks, especially when later on he starts making more encouraging remarks, basically encouraging the player to make their own mark in the real world, no matter how small, because even the smallest marks can have the biggest impacts. He pretty much disregards the fourth wall on a regular basis, even to the point where he starts speaking as his own voice actor rather than Sam in one instance. Naturally, unlike Elohim, Sam doesn't take his job seriously at all - he laughs at the player when he dies for the first time, outright encourages the player to climb the tower for a laugh, and even complains about Croteam using Ancient Egypt as a setting again.
THE TALOS PRINCIPLE LOCKED FROM THE INSIDE SKIN
And now there's a DLC pack that adds in an option for a Serious Sam skin for your character (as well as shades!), and an option to have Serious Sam's voice replace Elohim.
One of the spontaneous holographic replays you witness uses the same sound effect as a Beheaded Kamikaze from the Serious Sam games.
World B is reminiscent of the first game's Ancient Egyptian setting.
More than a couple of the secrets/easter eggs reference Serious Sam.
Even the Messenger worlds contain easter eggs! The one found accessed from hub contains, hidden on a cliff, a cage with a monkey and an infinite symbol inside.
Using a hidden telescope in B5, you can see a sunken Statue of Liberty in the distance.
Completing it leads to the One Ring underwater.
When exploring the third hub in the DLC, you may find a Message in a Bottle that it's part of a Scavenger Hunt.
In the DLC puzzle "Goliath", you can find some apparently broken QR codes, that read when using an external QR reader: "Happy, happy, boom, boom, swamp, swamp, swamp".
Congratulations! You have just vanquished an evil serpent-god with your bare hands! Unbelievable, isn't it? (Yeah, right.
THE TALOS PRINCIPLE LOCKED FROM THE INSIDE ANDROID
You are an android who can literally dream of electric sheep.
THE TALOS PRINCIPLE LOCKED FROM THE INSIDE HOW TO
Requiescat in pace." It's a hint for how to reach the last star: climb out the window, shimmy across some platforms, and walk off a ledge.
THE TALOS PRINCIPLE LOCKED FROM THE INSIDE CODE
Before entering the puzzle on the tower's fifth floor, there's a QR code that, when scanned, reads " Nulla è reale, tutto è lecito.
You can use it to cut some vines in the hub to find a hidden QR code that reads "You are not here" (a reference to Antichamber).
Speaking of Minecraft, behind the walls in one of the levels is a diamond sword from that game.
Within one of these crates hides a cat you can free, which alters the "Free Will" ending slightly.
There's a hidden room in level B7 with a number of boxes from various games, including a crate from Serious Sam, a cube from Minecraft, and the companion cube from Portal.
To get ENTRY GRANTED to A̶r̶s̶t̶o̶t̶z̶k̶a̶ the secret, you need to find a green stamp. revealing a big red ENTRY DENIED stamp that may look familiar to anybody who's played or watched Papers, Please. When you activate the wall with the secret, it slides down.
In area B4, there's a secret area right behind you at the start.
Perhaps I have some Russian in my lineage."
One of the messages you can paint on walls (if you've completed the Sigils of Elohim free promotional game) is "These sigil locks come naturally to me.
The design of EL's racks brings to mind IBM's Blue Gene supercomputers.
When exploring the beaches of level A7, you can find a terminal whose screen shows the Blue Screen of Death.