I am still pretty sure this arg still hasn't been fully solved, it ending in just a badge makes no sense to me.
I believe there really wasn't an actual ARG planned. Reddit thought that the barcode was somehow related to a bigger mystery to match the noir theme of the comic. So a group or someone at Valve saw this and decided to have some fun. On 24th day they injected "nothingtoseehere.js" into the store page. As evidenced by Archive.org:
https://web.archive.org/web/20151223002220/http://store.steampowered.com/app/730/
^ Counter Strike: Global Offensive (December 23th)
https://web.archive.org/web/20151225203420/http://store.steampowered.com/app/730/
^ Counter Strike: Global Offensive (December 25th)
(Archive.org doesn't seem to have 24th day after the 10 AM PST update. So 25th day is all I can show).
If ARG had been planned, it would've been there from day 1. They removed the barcode from comic's frontpage on 24th day as a hint. People already knew that the barcode was related to Hitman somehow. Not sure if anyone took a look at the source code before 24th day, but there wasn't anything (obviously). So someone else who knew about this had to point it out. A password prompt could be activated when typing "SEARCH" on the game search bar.
When the barcode is used as a password for Hitman 47's store page, it gives "ic/4f21ca7". It served as a tutorial where to input clues to, as in:
http://store.steampowered.com/wintercomic/4f21ca7
This link contained a WAV file mentioning they were on the right track and tomorrow they would have to search again. By tomorrow, it meant the new page in the comic on 25th day, where Jake is staring at the huge billboard. This had another hint: "The Blitzen Incident" (However, as these comics were done long before the actual idea of an ARG became into fruition, other language versions didn't contain most graphical texts on them).
Searching "The Blitzen Incident" or just "Blitzen" would lead to Grim Fandango page. People would have wait for the next page in order to proceed as it contained the password: "Hot toddies and maseratis" for Grim Fandango's page and this would lead to Undertale.
After this, people would have to obtain another password from another clue. This clue wasn't hidden in the comic, instead it was in one of the trading cards. Which was Trading Card 6: Sending a Message, it had text resembling to a hex string "794E2CEA991". However, it was one letter incomplete (due to how hex strings are formed). The last letter was an "F" as in sixth letter of the alphabet, as this was the sixth trading card. Now that it was complete, it could be used on the wintercomic site to access another WAV file:
http://store.steampowered.com/wintercomic/794e2cea991f
This WAV wouldn't make much of sense as it used
military language. Once it was deciphered, it revealed letters which could be arranged as "BULLET HELL". This was the password for Undertale's page which would lead to Ikaruga.
The last password was hidden in Trading Card 7: Taffy. It had 4 weird symbols which originate from a very old game,
Dungeon Master. These symbols would spell out the words "LO YA GOR KU". It could be used on the wintercomic site to access yet another WAV file:
http://store.steampowered.com/wintercomic/loyagorku
Unlike the two other WAV files before it, this was completely in Japanese:
「忘れられた道が障害と報酬に繋がります。」
"Forgotten road leads to (both) obstacles and a reward."
It's unsure what "forgotten road" meant, but it could've been the "search" command on store page as it brought many obstacles (password prompts on each store page) and a reward at the end (the badge). Analyzing this WAV with a spectogram would reveal letters: "1v7531"
Using this code on Ikaruga's store page would activate the Red Herring badge. Red Herring is really something that draws attention from the real issue, hence its description being "ARG?" to hint there was no ARG at all and it was distraction from the comic.
nothingtoseehere.js was created on 24th day, barcode was removed on 24th day from the comic, WAV files have a header which state they were also created 24th day. The store page code doesn't work on all browsers and certainly not in Steam client, meaning it was made in a rush and wasn't properly tested. Finally, the badge's image was taken from 25th day's comic page which had a red herring and the image had leftovers from it.
The whole reason why people went crazy over this, was because they skipped some clues (sequence break). Then they discovered older clues on a later date and found "BULLET HELL" as one of Undertale's genres. Didn't help that some "symbols" were from "that-one-game" leading to more speculation.
It became as real when enough of weight and faith was put into it. If something comes up on a later date, it's because it already manifested into its own thing.