Possum is not an official Google update: Google never confirmed it. On September 1, 2016, the local SEO community reported a major shake-up in local pack and Google Maps rankings, documented most thoroughly in Joy Hawkins' analysis for Search Engine Land. The name came from local SEO consultant Phil Rozek: many owners thought their listings had vanished when they had only been filtered, so the listings were "playing possum".
What changed is known only from that community coverage, not from Google. The shake-up affected local results (the local pack, Local Finder and Maps rankings) rather than organic listings, and the most-reported behavior was a stronger filter: businesses sharing an address or a category with a competitor were far more likely to be filtered out of the local results. The searcher's physical location also appeared to matter more than before, with proximity exerting a stronger pull on what the local pack showed.
The scale was measured after the fact: a study Joy Hawkins ran with BrightLocal, covered at Search Engine Land, tracked 14,242 keywords across 1,307 businesses and found 64% of local keywords saw some ranking change between August 31 and September 7, 2016, while organic results were largely untouched. Because Google never acknowledged the update, there is no official description of it and no official recovery guidance; everything on this page reflects the local SEO community's testing and coverage.