Half Life 2: Episode 3, Portal 2 secrets

Portal 2 has a few secrets hidden with its levels; there are some that aren't all that cool, but most are mysterious and need to be solved. One is of the Rat Man - the guy that left messages telling you that the cake is a lie, and there is a strange message that makes weird sounds. Another is of the Borealis itself, or at least the word, anyway. The final is Adam Foster, a mod maker in the Half Life 2 community, has a secret developer commentary hidden in a strange room; it is another strange message.