Updated April 4, 2017 at 12:36 p.m. EST: The Defenders teaser has been officially re-released on the Netflix show's social media. It confirms the August 18 release date.
Take a look:
Original Story: A teaser clip for Defenders has leaked after Marvel/Netflix posted the video and then quickly made it private. The brief clip, released under the title "Midland Circle Security Elevator B" shows the loose-knit team casually riding an elevator through the lens of a security camera, while the voice of Sigourney Weaver's villain Alexandra says "Going down..."
Here's a mirrored video (via ScreenRant):
The video also seems to indicate a release date: when Jessica Jones disables the camera, the timestamp freezes at 08:18:20:17 - or, possibly, August 18, 2017. A url found in the video also leads to a fake New York Bulletin article related to the characters of the show, which at the very bottom confirms that as the release date for Defenders.
It's unclear why the video was made private after briefly listing it; whether it was a mistake or a marketing ploy, or if the information contained in it is accurate. If so, Defenders will premiere on Netflix on August 18.