This page lists the Easter eggs in HITMAN™.
An actress playing Isabella Caruso, the mother of Silvio Caruso can be seen during the mission. If the player listens carefully, they can hear that she is talking about her two sons.
Additionally, an actor playing Thomas Cross, the father of Jordan Cross, can also be seen. He is addressed by (first) name during a discussion.
The NPC names can only be seen during the contract creator tutorial.
On the worktable near the mechanics, an IKEA instruction-book lies open. The book shows that the entire ship is one big set sold by IKEA.
During The Final Test, Jasper Knight can use the alias "Blind Tailor" if he is tricked into moving to the communications room. This is likely a reference to Tommy Clemenza, the blind tailor from Hitman: Absolution.
The Return of Allan
Some of the maps found on the The Final Test feature another variation of the now infamous "Allan please add details" phrase. "God Damn! We still need details here Allan!"
Other mentions of Allan appear on sticky notes throughout the game.
One of the catering vans has the logo of Mini Ninjas, another game by Io-Interactive.
One of Dalia Margolis's model spys, Martha Herris, might be a reference to Mata Hari an exotic dancer who was executed for accusations of being a German spy. Firstly, their names are very similar to each other, and Hari lived some of her life in Paris, where the mission takes place.
When complaining to his staff about the food not being out, Novikov mentions that he got them from Lestrades in London. This may be a reference to the character from the Sherlock Holmes novels Detective Lestrade who works for Scotland Yard.
Harry and Marv
The targets Harry "Smokey" Bagnato and Marv "Slick" Gonif are based off of Harry and Marv from Home Alone.
Shotgun and Roses
There's a shotgun lying on the ground with roses, as a reference to the mission Traditions of the Trade.
Agent 47 can summon a Kraken by ringing the bells on a faraway sailboat. Hitting the four bells in order from lowest to highest (notes, not height) with any sniper rifle causes tentacles to rise from the ocean and pull the sailboat underwater. After this is performed for the first time, the player passes a challenge and earns an achievement/trophy.
Super Mario Brothers
Agent 47 can come across two plumbers on the opposite ends of the town. One of the plumbers, Mario Salvatore, is wearing red overalls, and walks around the sewers near the marina. The other, Luigi Salvatore, who wears green overalls, can be found fixing pipes a few stories underneath the mansion kitchen.
Given the colors of their clothing, the Italian setting and the names that can be seen in contracts mode, the two plumbers are a clear reference to the Super Mario series, the main characters being Mario, and Luigi.
If the player sneaks into Rocco's apartment (the one above the ICA safehouse) they can see a screenshot of Hitman: Sniper Challenge on his monitor. Rocco is played by the man who had the #1 ranking in the challenge, and got the role as a reward.
Wearing the plague doctor disguise on the crypts underneath the church causes bony hands to pop from certain walls as the player walks past them.
After the release of the Sapienza trailer, many (jokingly) misheard Agent 47's line "Check Please" as "Jack Please". Indeed, the waiter's name on the released level is Jack Please.
Wearing the plague doctor disguise and trying to enter the mansion grounds will prompt the guards to mention Assassin's Creed by name when stopping you from entering the mansion.
In The Icon, there are 2 NPCs talking about replacing Dino Bosco for an actor named "Dave Bateson" for the main role of the movie, The Icon. It's a reference to a part of Hitman: Absolution's development, where William Mapother (Dino Bosco's voice actor) nearly replaced David Bateson for the role of Agent 47.
When you shoot all 7 balloons scattered around Sapienza, a balloon will appear being held by 47.
When you release the moose in the consulate and then shoot 4 bells scattering Marrakesh, the helicopter in the sky will rapidly be dropping a giant moose, killing any NPCs below.
When Claus Hugo Strandberg is killed, the Swedish flag will be removed from its pole.
Agent 47's Barcode
Referencing the first Hitman game, Hitman: Codename 47, there is an Easter egg on one of the logo's for the restaurants, that's supposed to be a high quality re-creation of Agent 47's barcode from Hitman: Codename 47.
Elephants Never Forget
In Ken Morgan's penthouse, there are 18 miniature golden elephant statues lying around. Shooting them produces loud, haunting animal roars. If all 18 are shot, there is an even louder roar, the player is awarded an achievement/trophy, and the large elephant statues at the front of the hotel begin to bleed from the eyes.
A guest of the hotel can be heard telling his friend that due to his popularity, everybody loves him, and thus could become a president. He then mentions how "he will make this country great again".
Wearing the Exterminator disguise, speak to the hotel manager to start an evacuation, go to the music crew studio, and play the keyboard (Blend In as Exterminator). Now, move to the exit with the motor boat, located near the default starting point of the mission, and you should see a giant cockroach in the background.
The Puddle Pile/The Golf Ball
Two militia soldiers can sometimes be heard discussing two events. One is occurred at a fashion show in Paris, where 70 people were killed due to faulty wiring in a puddle of water. The other one is in Sapienza, where someone was found dead under the well. This is a reference to Achievement Hunter and their video "Things to Do In Hitman - Puddle Pile" and "Hitman Golf - Hole in One" by TheKotti. 
Donald Trump (#2)
At the shed by the house where the Hallucinogenic Drugs is located, one of the militia soldiers stationed there may mention seeing a "billionaire presidential candidate" with a "famous hairdo" on television. This is a clear reference to the 2016 Republican presidential candidate and later president, Donald Trump.
The Wicker Man
When you disguised as a scarecrow and then using the sniper rifle to shoot 4 bells in an order, you can set any NPCs up in flames when you go near them as long as you still wear the scarecrow outfit.
Take the quad bike key, hallucinogenic drugs, and 10 apricots. Go to the room where there're 2 NPCs playing games, there'll be a prompt to "feed" the animal toy; do that 10 times and then take the quad bike exit. The quad bike is replaced with the car toy 47 will be shown exit the mission using that car. Note that this easter egg can only be triggered in Contract, Escalation and Elusive Target Mode.
The Motorcyclist disguise in the morgue is a reference to the uniform in the Kill Bill movie.
Dancing Game High Scores
There're familiar names on the high scores board of the dancing game like Aggemam, Rocco, White000 and HHCHunter (moderators of Hitman Reddit), A.Smith, Matsuda (CEO of Square Enix), etc.
If Agent 47 plays the dancing game, he will awkwardly stumble through the steps before giving up, unless he is wearing the ninja disguise. While wearing the ninja disguise, Agent 47's score will earn first place on the scoreboard, and his username on the board will be registered as "Barcodeman_47".
I Need To Use The Bathroom
Head to the lobby bathrooms. If you have not changed disguises and are wearing the suit, the unoccupied toilet should gain a button prompt to "Request Permission". If pressed, Agent 47 says, "I need to use the bathroom", a line from Codename 47 famous for its awkward delivery.
- When KAI is sabotaged, one of her lines when she's corrupt has her say, "Oh, I hope there will be cake! There will be cake, won't there? I was promised, you see." This is the reference to GLaDOS, the main antagonist of Portal.
- When you trigger the pushup contest (disguised as guards) and win, sabotage KAI, and then go back to the push up room, there'll be an AI named Ayakashi in shape of the ball fall from the roof. He is a reference to Wheatley of Portal 2.
- If you manage to push Ayakashi off the clip near the bike exit and then go back to KAI server room, she'll sing. This is a reference to HAL 9000 of 2001: A Space Odyssey.
- The "Daisy, Daisy" challenge when you assassinate Erich Soders with the robotic operating arms after you sabotage KAI is also the reference to HAL 9000.
When you climb the roof as the ninja, there'll be a lantern on the roof. Follow the hidden path (the lantern will disappear when you go the wrong way) on the roof to reach the lantern, there'll be a prompt that makes 47 do a ninja pose. A Godzilla will appear when 47 does the pose.
When The Curator is talking to the phone saying about obtaining rare-coloured eyes, he calls the person on the phone "Hannibal Chew", Hannibal Chew was the character in Blade Runner who creates artificial eyes for the androids.
The director's name is Akira Nakamura. Akira Nakamura is the name of a professional Japanese baseball player. There's a baseball player disguise in the director's room.
When you shoot all 10 mini ninjas toy in Hokkaido, the volcano will erupt.
Kane and Lynch
In the garage, next to the gardener fixing the skidoo, there is a poster for the game Kane and Lynch, another game made by IO-Interactive.
Hair on fire
In the hospital section of the level, you can find two doctors having their break. Let them talk for long enough and the lady will start saying how the man's hair looks so dry, that it looks like it'd catch on fire. this may be a reference to level 12 in Absolution, in which you could set someone's head alight.
We Meet Again
In the morgue, you can find an electronically locked refrigerator that can be unlocked with keycard from the hospital director’s office (or a disposable scrambler). Upon unlocking it, you can find an old ally/friend of Agent 47, Carlton Smith. He will then begin talking to you about what he was doing there and how he got locked up.
If you choose to start in Tobias Rieper’s Room as a location in the loadout. To the left side of your room. Another room will have a man wearing a cowboy hat if you manage to get in his room you can kill him and disguise as “Dexter”