I noticed this as well. Google does not need this office space right now. They have not been hiring in Austin at all, really. (More because of not hiring much in general than anything Austin specific).
I wonder if they might try to consolidate existing office space (one building over the Target on the east side and the building on 2nd St next to this one) into this new building, but that's pure speculation. Perhaps they simply figured that they were already paying to lease the building, so why not get some free advertisement from it
.
I feel like one or two of these buildings would have been subleased if there was a market for it, but that probably would require Google to take a massive loss.