In test chambers 2, 6, 7, and 9 in the Portal add-on, it is possible to briefly spot Doug Rattman in various areas before he runs away.

In the Portal series, Doug Rattman is a scientist who worked for Aperture Science and is an integral part of the series lore. He is alluded to but never actually seen in the first two Portal games (though he did star in a promotional comic for Portal 2). This is technically the very first time players have physically seen Rattman in any official Portal game.

These Easter eggs were not discovered until over a year after the game had launched.