In addition to the "8" and "X" banners referring to the upcoming releases of iOS 8 and the next version of OS X, Apple has additional banners at WWDC that are still under wraps.
Apple has opened the ground floor of Moscone Center to early registration, but isn't letting anyone upstairs. It's the ground floor where the main banners (above) are hanging.
In the two levels above, Apple has additional banners, but they're still covered up. While the floors aren't accessible to outsiders (or even registered attendees yet), is has been possible to catch a sneak peak in previous years by trespassing on the top floor of the Fifth and Mission parking garage, which is technically closed.
The tops of additional banners on the higher floors are partially visible from outside (above, and enhanced), but those banners are still covered up.
In previous years, Apple has hung both unobstructed banners for supporting technologies like AirPlay, and additional secret banners that remained cloaked until the Keynote disclosed new initiatives. It looks like 2014 will be a year where a few more secrets are revealed.