Undetected is an indie stealth game heavily inspired by the classic Metal Gear Solid game, using top-down view and fixed cameras. During your playthrough, you will encounter locked doors, which require different levels of keycards. The keycard level you have is shown below the health bar on the top left corner, and the level required to unlock a door is painted on the door.

Most of the achievements (8/11) for this game are story-related, so I won't really go into detail with those. The other 3 are hidden, but 1 is difficulty-related and the other 2 are like finding Easter eggs. They're not that easily missed, but could be if you don't know what you're looking for and the backtrack can be frustrating if you missed it. Here, I hope to explain them.

These ones are story-related, so you don't have to worry about missing them. You finish the game, you'll have them all.

Chicxulub: Find the Chicxulub Power Station

Gloria: Discover what happened to Gloria

Hacker: Hack the mainframe

The Nose: Encounter Jimmy McCoy

The Mind: Encounter Evelyn Johnson

The Body: Encounter Agent C-4815

The Lead: Encounter Hunter Patton

A New World: Finish the game

Absolute Master and a Real Hero: Finish the game in Mexican Extreme difficulty
The hardest achievement in the game, not recommended for first run unless you hate yourself. This mode is the maximum difficulty. Not only the enemies are more aware and you take more damage, if you got detected, you fail and your save gets deleted.

YEET: The Chameleon must be destroyed
For this achievement you must shoot a poster showing The Chameleon, a reference to the developer's previous game.

How do you get there? Not long after meeting Gloria (and getting the story achievement) she will ask you to go to the admin building. From the basement you'll ride a lift up and enter a lobby. You'll know the lobby because a text pops-up on your screen, and there's the station's logo in the middle of the floor. The exit of this level is on the bottom left corner, up the stairs and into the hallway I added a green arrow on.

A few rooms later you'll enter a room with a security camera. Darius Davies, your radio ally warns you about it. Near the exit of that room is a door that leads into the bathroom. Pull out your gun and shoot The Chameleon's poster with a bullet of your choice. Preferably the rubber bullet since its infinite.


Wallbreaker: Break a wall to save a carnal
For this achievement you must rescue a prisoner who is trapped behind a cracked wall. I don't know what reference this is to, maybe nothing and just a fun little secret since for paying attention.

How do you get there? Some rooms after the one with the bathroom poster, you'll come upon an elevator. Ride it up and you will be in the Servers floor. Your mission goal is to enter the red door and hack a server behind it, but you don't have a 3rd level keycard yet. So head up the only possible path, the hallway leading to the servers on the left. Later on you'll make a roundabout and re-enter this room from the hallway to the right like the arrows indicate. If you already have the 3rd level keycard you can just go straight to the right and ride the elevator in the next room.

Later you'll be in this tight hallway after exiting the elevator. See that cracked wall? Shoot a rubber bullet at it and free the prisoner. The door across from the elevator with the number 0 leads to the Offices section, and before you enter Darius will warn you about a prisoner with a 3rd level keycard. You can get this achievement before or after that, the office section only leads to the rooftop. But you'll have to backtrack and use that elevator again, and use the shortcut that leads you back to the server you need to hack before you can proceed to the rooftop to progress the story.

So in conclusion, the secret Easter egg achievements takes place between the unlocking of "Gloria" and "Hacker" achievements so you don't have to worry about it in earlier or later levels since there are no others, only story and difficulty. You can potentially miss these achievements if you progress too far, or have annoying backtracks, therefore I recommend keeping an eye out once you get to the lobby.

Credits to JacketAU and Underdrill for finding out the requirements to unlock the achievement. I found it through a Discussion forum, link to discussion here.