Prototype Easter Egg - The Cure May or May Not Be Cake

In free-roam mode (after the completion of any difficulty setting in story mode) there are many billboards of cure and military advertisements. About three or so blocks north of the time square marker on your map, there is a large billboard that reads THE CURE and underneath with spraypaint/ blood is writen, IS A LIE. followed by no cure is coming. This is an obvious reference to Portal's THE CAKE IS A LIE. And the writing is also very similar.

(Thanks to my friend for finding this egg)