If you shoot an 'L', then an 'O', then another 'L' on the 'Gil's Lodge Motel' sign in Extiction mode the Golden Aliens easter egg will become unlocked. This feature lasts about 90 seconds and can be repeated.
This feature becomes an available option at the 'Main' menu when you start a game in Single-Player Campaign mode on the Regular or higher difficulty setting and then complete the 'Ghost Stories', 'Brave New World', 'No Man's Land', and 'Struck Down' missions.
In the mission "Stuck Down" there are some trophies in the locker room. One is shaped like a rupee and reads "Most bushes cut and pots smashed". Another trophy is shaped like a tardis and reads "Smallest exterior with the largest interior".