Door 1/Cryptic Message 1:
There is a radio on the cashier's desk that plays Morse code. When translated, the message reads: "DIMENSIONAL".

Door 3:
Interacting with the basketball art triggers the instrumental version of "Soulchef - Write this down."

Door 5/ Cryptic Message 2 :
Behind the cabin, there is a texture of HL1 GMAN's face. However, nothing happens when interacting with it. Upon further exploration, a radio can be found behind the teleporter. When teleported once and interacted with, the radio plays a Morse code message. After translating the Morse code, we obtain the message: "EXPLORER".

Door 6:
After collecting five books, a teleporter is activated. Entering it transports the player to a darker room with only a table containing a top-secret file. Shortly after, an explosion occurs, revealing four Combine Elite in the rear view. The player is then teleported back to the lobby room.

Door 7/ Cryptic Message 3 :
Similar to Door 1, there is a radio on the cashier's desk that plays Morse code. Translating the Morse code reveals the message: "CONTROLLING".

Door 9/ Cryptic Message 4 :
By bringing the blue stuff to Barney, he expresses gratitude and returns to making a map, as he did last year. In the secret room, there is a Steam link that redirects to a map from the TF2 Workshop. Next to the computer, there is a radio that plays Morse code. Translating the Morse code reveals the message: "KINETIK".

BONUS:
In the "This will be my next place." video, a Morse code can be heard. Translating the Morse code reveals the message: "I AM RUNNING OUT OF TIME".

BONUS 2:
In the third picture of the addon, there appears to be a white blank image. However, by using a photo editing software and making adjustments to the picture, a secret message can be discovered: "I WILL END THIS".

Door 1 isen't correct

Rewritten.

Door 10:
A hidden message can be revealed on the wall by using a lamp with caution. Begin by reading the text adjacent to the cart and proceed by walking towards the right until reaching the lantern. The message states: "CHRISTMAS IS ONLY A MEANS TO AN END TIME."

Door 12:
has a QR code that sends you to a yt video where a guy is filming you and he gets killed at the end

Lobby (Cave):
In connection with the Door 12, after watching the video accessed through the QR Code on Door 12, returns to the cave and goes to the exact location where the video was filmed, bloodstains can be found on the snow. These bloodstains are from the man who was killed at the end of the video.

Door 13/Cryptic Message 5:
A radio emitting Morse code can be discovered under a stack of crates. Decoding the Morse code reveals the message: "ENERGY,". :

Door 15/Cryptic Message 6:
When enter the room, turn on the flashlight and head to the left to find a radio playing a Morse code. Decoding the Morse code reveals the message: "NAVIGATING".

Door 19/Cryptic Message 7:
By taking the third teleporter to the third row on the right, you will be teleported from the roof of the map. Behind, there is a radio playing Morse code. Translating the Morse code reveals the message: "INTERDIMENSIONAL".

where each teleporter takes you:

L1: 3rd left teleporter
L2: 2nd right teleporter
L3: out of order
L4: room 18
L5 8th left teleporter
L6: 1st left teleporter
L7: room entrance
L8: room 5

R1: 4th left teleporter
R2: 2nd left teleporter
R3: right side of the 1st lobby
R4: room 15
R5: spawn point
R6: 2nd lobby roof
R7: 7th right teleporter
R8: 1st left teleporter

Door 24/Cryptic Message 8:
By taking the door to the right and crouching, there is a radio playing Morse code. Translating the Morse code reveals the message "DOMAINS".

so on 25th door after beating combines we get a QR code that leans to a mediafire link to download here.zip and it's password locked but it's unclear (for me at least) what we should use decryped messages for :/ (i've tried to use first letters each messages in CAPS or normal texts but none of em worked)

schizo moment

Lobby:
From behind the lobby, in the underground area, there is a retro disk that can be found and played to play an SSTV audio. Using an SSTV Image Decoder software to play the SSTV audio reveals an image of the profile picture of deckenid, the creator of the map. This is remind of Valve's use of SSTV audio in the radios from Portal, which led to an ARG for Portal 2.

Originally posted by OctopusHorizon:

Door 1/Cryptic Message 1:
There is a radio on the cashier's desk that plays Morse code. When translated, the message reads: "DIMENSIONAL."

Door 3:
Interacting with the basketball art triggers the instrumental version of "Soulchef - Write this down."

Door 5/ Cryptic Message 2 :
Behind the cabin, there is a texture of HL1 GMAN's face. However, nothing happens when interacting with it. Upon further exploration, a radio can be found behind the teleporter. When teleported once and interacted with, the radio plays a Morse code message. After translating the Morse code, we obtain the message: "EXPLORER."

Door 6:
After collecting five books, a teleporter is activated. Entering it transports the player to a darker room with only a table containing a top-secret file. Shortly after, an explosion occurs, revealing four Combine Elite in the rear view. The player is then teleported back to the lobby room.

Door 7/ Cryptic Message 3 :
Similar to Door 1, there is a radio on the cashier's desk that plays Morse code. Translating the Morse code reveals the message: "CONTROLLING."

Door 9/ Cryptic Message 4 :
By bringing the blue stuff to Barney, he expresses gratitude and returns to making a map, as he did last year. In the secret room, there is a Steam link that redirects to a map from the TF2 Workshop. Next to the computer, there is a radio that plays Morse code. Translating the Morse code reveals the message: "KINETIK."

BONUS:
In the "This will be my next place." video, a Morse code can be heard. Translating the Morse code reveals the message: "I AM RUNNING OUT OF TIME."

BONUS 2:
In the third picture of the addon, there appears to be a white blank image. However, by using a photo editing software and making adjustments to the picture, a secret message can be discovered: "I WILL END THIS."

Door 9 isen't correct