In the support document I linked above there is a section on privacy.
The pertinent part:
The sync event does not let us know whether you performed the action on a Plex Media Server library item, a Discover page, or through a Movies & Shows streaming title. Activity from any of those would look identical. So, there is no way to know whether something being “watched” occurred because you went and saw it at the theater and then marked it on the Discover page when you got home, you watched through a personal Plex Media Server library, or anything else.
