So this addon's description contains something interesting. A youtube video with strange noise and 3 Base64 messages.
After decoding those messages i've got:

RklORCBUSEUgQ0FU - "FIND THE CAT"
T1IgU1RBWSBIRVJF - "OR STAY HERE"
QXJlIHlvdSBzdGlsbCBoZXJlPyBXaHk/IA== - "Are you still here? Why?"

The strange youtube video with noise is a sstv message. After decoding it i've received this:

https://imgur.com/a/rqjoimw

It's a picture of a strange map, a platform with tiles in the centre of nowhere with hl2 skybox (i suppose it's skybox named "sky_day01_01", which is default skybox for hammer editor) surrounded with some sort of metal bars fence (a hl2 model) and a blue metal chair in the middle of the platform. And on top of that there's a strange red text in the centre of the image "zbbac112"

I don't know what it means. Googling doesn't help.
I could use some help if someone is as curious as me!

I'll post updates on searching the easter egg, in case I find something else.

Update №1 [jun 5, wed, 13:45 GMT]

I've decompiled this map and scanned it with VIDE. It turns out this map contains "matsys_regressiontest/background" material, which is an image of a cat. That's why decoded messages said "FIND THE CAT"
I've tried marking the textures with face edit tool, and it says that 4 faces have this material, so, probably, there's 4 instances of this texture somewhere on the map. I'm currently searching for it.

Update №2 [jun 5, wed 14:19 GMT]

I've found the cat.

https://imgur.com/a/s3cCdr6

Image contains the location of the cat texture, and it seems that there's nothing else that could be interesting in this map. The 4 instances were 4 faces on one plane, they were divided because this some sort of a cube figure is made with clipping tool and it just cuts the brush into the segments so the middle parts are deleted to form the figure.

Also one thing, that I don't understand, how the map works with leaks?
It is not sealed and it should leak, but it isnt... Very strange case. Maybe it's decompiling artifacts.. But if you just run the map and fall into the void, you will fall infinitely till the z position equals to -131003.20 for some reason (probably source engine jank). After dying player_entity will fall further into the void, it's just when you are alive it's stopping you at that coordinate for some reason.

I've also noticed the last image on this map's workshop page is a decoded sstv image.

https://imgur.com/a/EjotHc8

I'll think what to do with it. I've archived this map's workshop page in wayback machine in case it changes.

Update №3 [jun 5, wed 17:23 GMT]

Addon description got updated. Now it has a new Base64 Code which leads us to the message:

"RAAAAG8AAAAgAAAAeQAAAG8AAAB1AAAAIAAAAG4AAABlAAAAZQAAAGQAAAAgAAAAYQAAACAAAABsAAAAYQAAAG4AAABnAAAAdQAAAGEAAABnAAAAZQAAACAAAABnAAAAdQAAAGkAAABkAAAAZQAAAD8AAAA="

D���o��� ���y���o���u��� ���n���e���e���d��� ���a��� ���l���a���n���g���u���a���g���e��� ���g���u���i���d���e���?���

"Do you need a language guide?"

Also from what I've noticed, the sstv decoded image disappeared, and youtube link has a little note on the side

"Vunči Ussuãstã. Sãřme."

I think it's czech, atleast google translator says so, it translates as
"Against Ussuãstã. Let's eat."

p.s the little side note already was there, i just checked at the 14:19 snapshot, so description update only brought new base64 line and removed decoded sstv image

I have no idea who is "Ussuãstã" because googling doesn't give any results.
BUT!

I haven't discussed youtube video title, "Tãjo ychtããstã" as the google translator can't translate this properly, as it detects the language as "West Frisian". And then if we translate "Vunči Ussuãstã. Sãřme." from West Frisian to English it appears as the sentence "Vunči Ussuãstã. Same". Still I have no clue what could that mean. Google refuses to give me any search results. But it's clear that "Vunči Ussuãstã" is someone or something that is somehow related to this map/addon.

That's all that I've noticed from the update. The addon itself didn't got any updates, neither got the youtube video.

I've archived this state of this map workshop page once again.

Update №4 [jun 5, 21:34 GMT]

A new youtube video link was put into the description. Once again a SSTV cipher. Decoding it lead me to this image:

https://imgur.com/a/TcFdHmi

It's an image of 2 white hands reaching each other, and at the bottom "U.I.L" which means "Universal Interworks Language", that is also showed on the picture, but barely visible because of my bad SSTV decode attempt.

This is getting interesting...

The video name "Sõkõãstã" is once again untranslatable.

Workshop page is archived. Also, previous video was deleted, but i have the mp3 file downloaded, as well as the new one.

Update №5 [jun 6, 14:25 GMT]

A new youtube video link was put into the description. Once again a SSTV cipher. Author of this ARG really likes portal ARG with SSTV messages..

Anyway, decoding this sstv cipher lead me to this image:

https://imgur.com/a/Pns15Lq

Image shows us a skeleton covered in blood lying on tile floor, same floor we have seen on the first decoded sstv image of a blue metal chair in the middle of nowhere.

Also, again, a note was left near the youtube link.

"unfortunately not every pilgrim lucky person"

Unfortunately, not every piligrim is a lucky person, I guess.

Do not know what could that mean, piligrim is a wandering traveler without any objectives, just traveling 'round the world.

Not trying to be a captain obvious here, but it's probably some sort of metaphor, like you should be aware of something..

About the youtube video title, it's "Fi wãyã lãssisvãstã" once again can't be truly translated through West Fristian, but google translator thinks that this is so called "Harshen" language, and if we actually translate it through this language we will get "It's better than the phone" which still doesn't make any sense to me to be honest, but i've also translated all of the previous titles with harshen language and this is what i've got:

Tãjo ychtããstã - "She came to Ychtããsta"
Sõkõãstã - "Sókõãsta" (bruh)
Vunči Ussuãstã. Sãřme. - "Vunči Ussuãstã. It's okay."

No idea what could that mean.. My guess is that "Ychtããsta" is someone like a monk or something..

I think this whole easter egg or an arg is just sneak peeks to SoyCow Boy's new map.
Anyway, that's it for this update, i still didn't check every single detail, maybe there's something else.

Workshop page once again is archived.