As I recall one of the first ever secret areas in a game was Warren Robinett adding his name in Adventure for the Atari 26000 because he wasn't allowed to disclose his involvement due to fears of headhunting.
And of course, you later got the notorious "You're not supposed to be here - Levelord" from Duke Nukem 3D.
The level designers for the Cryptic Passage expansion for Blood also signed their name in level vertices as well.