It seems that Sledgehammer Games wanted to spread the easter egg love beyond Call of Duty: WWII’s Zombies mode as players have now discovered a series of hidden gridiron balls which are tucked away in the game’s Headquarters social space.
There are three gridiron balls in total, and finding all three nets you an exclusive calling card. The first ball’s location can be seen in this recent tweet from Charlie Intel. The second ball requires some precise platforming up the scorestreak tower (here’s a video walkthrough)and the third ball sounds like it’s the most difficult one to reach, at least according to this Reddit excerpt:
“You start off inside the theatre/projector area on the left hand side on some rubble. This is the first tedious jump; it's like a sideways jump but you gotta push up against the wall at the same time to a small ledge thing. Eventually, you'll be on top of the building thing, this is where you head to the right and jump onto a small pillar and then jump onto a rubble wall thing. This is the second tough jump; ahead on the Main/Operations(?) building there is a small pipe sticking out the wall.
Carefully maneuver yourself to the "yellow" square pillar on top of the same wall your standing on and jump towards the pipe. From there do a slight jump up onto a green ledge and up onto the concrete roof. Here you'll walk to carefully walk all the way around the structure and eventually you'll enter the dark "lookout" point and the second ball is there accompanied by good Ol' Kilroy on the wall.”
Even if you don’t want to go for all three, each ball you find grants a medal and some bonus XP, so at the very least it’s worth getting the first ball and maybe even the second.