Oh that's cool. This happened to me a few years ago, we'd moved house in December and had no internet all Christmas/new year due to all the contractors being on holiday. I was emotionally scarred enough to still remember it today!

Maybe that behaviour's fixed now, or maybe I just hadn't found the setting mentioned in the comments below.

To answer your question, yes it appeared to be attempting to authenticate with their servers, even though my Plex server was local