GUYS so i do currently do not have a follower. and i was up near traitors post by windhelm. and i was fighting the bandits. and all of the sudden, the bandit furthest from me just fell over dead. so after i dealt with all the other bandits, i went to investigate. and there was an arrow petruding from her body. I was using a sword so it couldnt of been a misfire. and i looked around and i saw no one. i crouched and shouted and i remained undetected. so i walked a bit further and came across the monument where people met crossing over the border. there was a frost troll and i as i was fighting, when he had almost no health left, an arrow shot him dead.... i never found my mysterious follower.

Maybe Kynareth is your guardian angel.

Haha, that's pretty awesome.