The couriers are random ( some usually require certain actions in-game to occur beforehand, for example, the Dark Brotherhood.) and can spawn almost anywhere, as I have encountered them in the wilderness, travelling the roads and in cities. I have been attacked by Thalmor squads only just after leaving a dungeon, and challenged by a mage in Whiterun.

I would say that random encounters occur in areas of a certain category, but are random when spawning in specific locations within those categories.