Project Loon's balloons could not be more different than your typical party variety -- it's loaded with research equipment and LTE capability, providing high-speed internet connection wherever they go. Obviously, Google's X Lab researchers (the ones behind this crazy balloons-as-hotspot project) will want their data and expensive equipment back. So, they equipped their balloons with GPS and formed a special team to retrieve the floating hotspots when they land. Apparently, the researchers plan out when and where to land balloons for whatever reason (they mostly choose flat areas that are uninhabited but have decent road access), which the field personnel then seek out through their coordinates.
Since this process allows the Project Loon folks to clean up after themselves and reuse old equipment to save money, they take retrieval seriously. Once, team member Nick Kohli even traversed New Zealand's waters in a small fishing vessel for two days, in order to collect balloons that landed in the area. Since he's most qualified to talk about how retrieval operations work, you can listen to him explain it in detail and watch how it all goes down in the video below.