Another possibility for seeing barbs in a "no barbs" game that I noticed. When I got GS a week ago, I started fooling around with the worldbuilder. I've noticed that in addition to the civs and city-states spawning, many maps also spawn a free city. It's units, I believe, have the barb label. Along the same lines, cities that loyalty-flip are temporarily free cities, and their units may be labeled as barbarians.
I've never seen a free city just spawn from nowhere. I've only seen free cities when a city loses loyalty. And, those cities spawn free city units, not barbarians.