Just as a helpful tip: "inside the secret world" is an colloquial term for just "here's an inside peek at something not so widely known", not that it's literally "secret" and hidden.
Except what is in the article is widely known. The article only did a surface level overview which implies that the secret is the existence or concept of them. The article even eludes to this in saying that one of them was in an unmarked building.
These are widely known. The article is just using the standard annoying fetishism of Japan for clicks.