A few months ago there was a Portal update where after you beat the game once, radios start appearing in every test chamber. If you brought the radio to a certain location, it would start to play a different sound. Bringing all radios to their correct locations unlocks the Transmission Achieved achievement.
The various sounds the radios produced were found to contain hidden messages, such as images, or the login and password to a certain BBS (which then contained more clues). The Portal community had a field day figuring everything out. It turned out to be a sort of viral marketing for Portal 2.