As announced in the most recent Famitsu, Kojima Productions announced a URL that seemed like it would announce something else on May 18. Today is May 18.
That teaser has updated with what could either be the number 5 or maybe a block capitalized letter "S". The website has lighting and thunder and wind blowing noises as well as a countdown clock that ends in three days. Another announcement of another announcement? If so, that would make this whole affair an announcement of an announcement of an announcement.
Thanks treegreen for the tip!