What factors contribute to world completition?

I was recently running through Caledon Forest, where I had 100% completion, when I had a message "[some area name] discovered" pop up. This also revealed the area on my map. It wasn't a point of interest, just a part of the map. Since I already had 100% for the zone, how am I meant to know if I've found all these areas? Do they count towards overall completion, or do they not count for anything?

What factors contribute to world completion?

Those covered areas of the map count toward achievements (Kryta Explorer, Maguuma Explorer, etc.), but not toward World Completion.

The only things that concern your world completion percentage are the Points of Interest, Renown Hearts, Waypoints, Vistas, and Skill Points in the four WvW maps, and the main world (excluding Southsun Cove and Eye of the North). Dungeons don't count toward map completion, so you don't need to worry about those either.

Unfortunately, the only way to know whether or not you've uncovered the entire map in a particular zone is to thoroughly study your own map. You could also compare your map with one from the Wiki, if you are worried you might have missed something.