Two SEOs spotted Google testing a new design for Google Sitelinks. The new design has sitelinks that expand and show sub-sitelinks in a gray background area. Google tested something similar on mobile sitelinks a few months ago.
Here is a picture from +Moz:
Here is a picture from +Jon Cooper:
As you can see, clicking on a sitelink within a white background will drop down a gray background area with more sitelinks to be clicked on.