After obtaining all ability upgrades, Spoiler:Tatiana will congratulate the player by donning a party hat and blowing a party horn as fireworks explode in the background.
Contributed by ProtoSnake
There are collectibles in the game that are references to Bethesda Softworks' previous games. After finding them, they can be viewed in Sebastian's Room.

• Mysterious Mask - Corvo Attano's mask from Dishonored.
• Mysterious Machine - Panzerhund's model from Wolfenstein: The New Order.
• Mysterious Weapon Rebplica - Rocket Launcher model from Quake.
• Mysterious Bobblehead - Vault Boy bobblehead from the Fallout series.
• Mysterious Action Figure - Doom Marine/Doomguy action figure from 2016 reboot of the Doom series.
• Mysterious Symbol - Ouroboros symbol from The Elder Scrolls Online.
• Mysterious Mug - TranStar mug from 2017 reboot of the Prey series.
• Mysterious Toy - The Keeper toy from the game, The Evil Within.
Contributed by ProtoSnake
When starting New Game Plus, the player can trigger an easter egg by searching for three zombies hiding under three different cars scattered around the map (for this secret to work, those zombies must not get killed, knocked out or become aware of your presence). All three zombies must gather behind the Safe House in the northern part of the map, and then they each must be killed with a melee attack (again ensuring that they do not notice you, otherwise you will have to hide again until they go back to attacking the building). After killing the zombies, a gigantic crow will appear nearby, which will reward the player with 60,000 Green Gel when killed.
Contributed by ProtoSnake
NSFW - This trivia is considered "Not Safe for Work" - Click to Reveal
The enemy Laments had their nipples removed in the Japanese release for the PlayStation 4 & Xbox One versions of the game.
Contributed by ProtoSnake
At the start of Chapter 7 after destroying two of Stefano's paintings, Sebastian can backtrack to O'Neal's Safe House and trigger a secret conversation with him featuring the following pun:

O'Neal: I told you I wasn't going to leave this place. It's fucking EVIL out there.

Sebastian: That may be... Spoiler:But there's EVIL WITHIN, TOO...

Both characters then momentarily turn and look toward the player before O'Neal quickly ends the conversation.

A localized version of this secret for the game's Japanese release, which was released under the name "Psycho Break 2", also exists:

O'Neal: I rather stay here doing a DANCE seriously.

Sebastian: You're DANCING? Spoiler:That's the BEST BREAK though.../おまえかダンスだって? そりゃ最高にブレイクだな... 最高にブレイク (SAIKOU NI BUREIKU/PSYCHO BREAK)
Contributed by MehDeletingLater