Being sure that I never intentionally hurt anyone who wasn't hostile or a potential threat I decided to investigate. I noticed that one hit, stealth kills on opponents that still haven't gone hostile (even if they're a sure threat like wild beasts or bandits) sometimes are counted as "assault".
This behavior seems to be random, as reloading and replaying the same sequence another time may or might not make the assault counter advance.
I'm the only one who noticed this? :unsure: