User blog comment:Argorrath/Treyarch posts a message of zombies; possibly hinting to a DLC 4/@comment-5213593-20130804050953/@comment-3967954-20130804211143

Why does there even need to be a canonical explanation for this? You might as well go ahead and explain why there's M14s and MP5Ks knocking about in an abandoned cinema in Germany in the 1960's.