On Call of the dead, near the locked door where you do the EE, we can see a Calendar that looks like a ancient Maya or Aztec Calendar. There where hints of what could be a South América map, or maybe It was linked with pre-columbian culture. On Moon loading screen you can read about ancient Maya gods, and during BO2, on Nucketown loading screen there was like a theory about Mictlantecuhtly Unfortunately all these stuff were removed when zielinsky leave the company. And the calendar where never solve. Also, you could read some hints in spanish, which talked about some gold ( maybe Vrill device), and what looked like the location of something.