Chapter 6 houses many secrets. Scout for 'vitriﬁed' doors and use their intercoms to hear Cave. To get the 'Door Prize' Trophy, use all six doors (three in the '50s, three in the '70s sections of Chapter 6). Mystery: vitriﬁed means turned to glass... was whatever lies beyond turned to glass? How? Why?
Chapter 8, Chamber 15
The singleplayer and co-op games have no obviously entwined timeline, but it's safe to assume they're happening at a similar time. The cameo appearance of testing robot P-Body in Chapter 8 suggests it. After Wheatley gets drunk on power he starts to move chambers around to create extra challenges. Keep your eye on the torn hole as he rearranges chamber 15, and zoom in. You'll spot P-Body standing in front of a doorway. He does a little jig, then continues on his merry way.
Chapter 3, Chamber 17
The dev's commentary concerns the game - except in the ﬁnal room of Chapter 3. Inside a hidden space lies a secret node that transmits a slow-scan television (SSTV) blurt. Decoding it gives a detailed run-down of the Alternate Reality Game, run before launch, that led players on a real life/net quest to get Portal 2 released early.
Chapter 1, Chamber 4 / Chapter 2, Chambers 3 and 6 / Chapter 3, Chamber 12, 16 and 17
Valve released the comic Lab Rat to bridge the events of Portal and Portal 2. It focuses on Doug Rattman, one of the few Aperture Science employees left alive after GLaDOS's neurotoxin attack, who begins building dens in corners of the chambers - out of sight. Enterprising players can ﬁnd all six of Rattman's dens, each covered in scrawled murals and strange markings.
Both comic and drawings suggest Rattman knew about Chell and wanted to save her. There's an extra level to one of the dens: bring a radio inside the den in Chamber 6 and you'll unlock a hidden SSTV transmission that, when decoded, shows a companion cube on the moon. Hmm. It also gets you the 'Final Transmission' Trophy.
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