One of the really fun gags on The Big Bang Theory this year was the empty apartment downstairs. It was such an unexpected and well timed bit that it really worked. But some fans have been wondering if it was a set up for the next season, a hint that they would be getting new neighbors. Not likely. Executive producer Steve Molaro told EW.com:
Honestly, I pitched it as a ridiculous joke that I need to get out of my head. It was just a visual sight gag on the heels of their lab in the security building being cleaned out for them to then come home and the apartment to appear to be cleaned out as well. It was really nothing more than a joke I thought we would never even do because it sounded difficult to have to empty out our entire set. In fact, we did not empty out our entire set, we rebuilt the apartment on the stage empty.
They built a duplicate apartment for the gag, that's actually pretty good for such a quick gag. The bit only last like three or four lines (as you can see in the video below).