Well, me and some of my friends were going back through campaign on legendary and we were at the point of the level just after the phantoms blast the pelicans. One of my friends managed to jump up onto one of the perches where the Jackals snipe from and came across this....
My guess is that it is one of the developer's face that they thought would be funny to sneak into the game....
Here's the link to my file share if you want the version without a water maker....
FantasticDuck Screen Shots