Update: Don't get too excited: Valve says the Easter egg was just "a bit of fun."
Congrats to all who have found our Portal Easter egg on dz_blacksite! We’re just having a bit of fun and the Easter egg is not a product announcement. Enjoy Danger Zone!December 10, 2018
Original story: Valve has hidden a cryptic Portal message within CS:GO's new battle royale mode, Danger Zone—and some fans believe it may be the start of a new Alternate Reality Game, similar to the one that led to the announcement of Portal 2.
At the eastern edge of the Danger Zone map, Black Site, you'll find four numbered rooms. Room number three is boarded up—wink, wink, nudge, nudge—but you can get into it by going through room two. Inside, there's a bank of computers and a strange broadcast message containing a string of words and numbers. Those that first found it suspected it was a code deliberately put there by Valve and, sure enough, it hasn't taken long for players to crack it.
Reddit user GetSomeGyros has mangaged to decipher the broadcast (you can read their method here), and the words they came out with are: "This was a triumph. I'm making a note here: HUGE SUCCESS." As you may know, that's the opening line to Portal's famous end credit song, Still Alive.
What does it mean? For now, nobody knows, but fans are hoping it's the start of a new Portal-related ARG. The announcement of Portal 2 was preceded by an ARG in which Valve hid clues within radios in the first Portal game—to uncover the clues, players had to carry the radios to a specific point in the map. We'll have to wait and see whether this is something similar or just an Easter egg.
In this thread, Reddit user birkir has outlined other possible oddities in Danger Zone's map, including strange logos, signs, and security cameras. No doubt players will continue digging into what it all means—Valve News Network (an unofficial channel) uploaded this video summing up what we know so far.
Credit to Reddit user snaileny for finding the broadcast message and uploading it to YouTube in the video at the top of this post.
Hat tip, PCGamesN.