How does something that big go missing and how did he stay hidden in the first place?
Presumably he dove down into the Mariana Trench, which is the deepest known point on the planet.
After that he headed back out to sea, swimming west across the Pacific and passing Skull Island along the way, until he reached the Mariana Trench and vanished.
Godzilla appeared in San Diego back in July, just in time for Comic-Con where he had a hot new trailer to drop.