Apple will introduce new versions of iOS and OS X at its annual developer's conference.
Apple Likely Building Apple Watch Demo Area at Yerba Buena Center
According to the tipster that submitted the original photo, there are security guards wearing "Apple Security" shirts monitoring the construction site, all but confirming that the external addition belongs to Apple. The company previously built a similar structure outside of the Flint Center as a demo area for the iPhone 6, iPhone 6 Plus and Apple Watch, so an exterior demo area would not be unprecedented.
Apple sent out invites for its "Spring Forward" media event earlier this week, and will also be providing a live stream of the event on its website for Mac, iPhone and iPad. The media event will take place at the Yerba Buena Center for the Arts in San Francisco on March 9th at 10:00 AM Pacific and should be focused on the Apple Watch ahead of the wrist-worn device's launch in April.