Final Transmission

The last of the Rat Man's dens hides a radio playing a coded signal. In Chapter 2, Test Chamber 6, dig the radio out of the fallen rubble and - crucially - carry it with you through the Aerial Faith Plates into the hidden den. Bring the radio all the way in and it plays the hidden transmission (a picture encoded in SSTV tones). Don't put it down along the way or you'll have to fetch it again.