I was chatting with some of the Google people at the event, and they said that the balloons have been responsible for a few UFO sightings. In fact, when one of the balloons' transponders failed in flight, they were able to track the balloon by the UFO reports online! This happened while they were testing the balloons in California's Central Valley.

They said the balloons are expected to take two weeks to circle the globe. Given that they said they'll eventually stay afloat for 100 days, that gives them plenty of opportunities to bring the balloons down in a specific location in order to make reuse easy.