if any scout, worker, settler comes within attack range of a camp/barb unit, it is 100% certain they will get attacked or captured. I have also found it suspicious that barb's seem to appear near settlers in particular. i usually have a unit scout ahead 1 to 3 tiles to escort, but I have had a couple run ins where inconceivably my settler was captured on a hill and no barbs around whatsoever within sight and on the end turn being captured.
i've also had suspicious movements from barbs where they seemingly call in reinforcements ahead of my units after another barb group has been "shadowing" my unit. smart play? i dunno
i often find that workers, settlers seem to attract barbarians and they even follow them which is annoying as you try to avoid the buggers. I often find barbarians a fun addition to the game, and leaving non combat units un-escorted is at your own peril.