I killed target long distance with my silenced pistol. Nobody saw me. Guards were not alerted. However, I failed “no combat” requirement.
So what does it mean?
Does it mean I should not fire a firearm? Not even to shoot to disable the surveilance?
What are the rules to achieve “no combat” challenge?
Did the target die instantly, or did it take multiple shots? If so, the target went into combat mode before dying because it was not an instant kill. If they did die instantly, then something else triggered it, possibly a bug.
It was headshot, clean, I was well hidden, far away…
Again same thing happened 
It’s Whittletone Creek, and the target is near other guests. However, nobody saw me. Last time I was shooting from the window of an empty house, now I shot from the entry door, I was totally alone, unseen both times. Yet my “no combat” challenge failed.
It wouldn’t be big deal otherwise, but I play hard core and I have “Perfect run” option (other 2 selections were impossible to perform: SIlent assassin without firearms, and some other unrealistic challenge), so it is mandatory to perform.
The target was in public? They don’t need to see you to be in combat. Just raising an alarm is enough to count. Nobody can see you kill the target and they cannot find the body. Getting the guards to start looking for you because they found a dead body is the same as shooting in front of them as far as counting toward this restriction.
To me, I think it’s the case of other civilians that must have witnessed the target getting shot. Let’s say that you shoot someone from a long distance and there’s other people who witnessed the person getting shot, and there would be guards who also witnessed it to. The game probably sees it as if you’re going to enter combat if you stay at the spot where you fired your shot for too long since the guards would run to where the shot came from.
It means you’re probably gonna fail that objective. Never, ever, pick Perfect Run when it’s one of the objectives. Also never, ever pick Perfect Run in HC if one of the objectives has a fail state.
If an NPC (guard or not) witness someone getting shot, it will trigger “combat” regardless of whether you were spotted or not.
Sometimes simply killing a guard will fail “No Combat” though, so it must be a bug unless that’s intended!?
If a guard sees you carrying an illegal firearm or you’re spotted in a hostile area it immediately turns into combat.
Sometimes it’s not so clear though.
Maybe your target saw you take the shot before they went down, and they were near a guard, who they then alerted right before they died. And then said guard started a firefight. That might have initiated the combat?
I think IOI will have to tweak the definition of this objective. You can shoot someone long distance, but as long as anyone witnesses the event… I’m going to guess that by “Combat” it’s triggering (a) fight (for guards) or flight (for regular NPCs because they’re panicked) response.
“Combat” with this objective is when guards pull out their weapons to search for you, even if they see something super suspicious like a dead body or someone being murdered.
Whether they see you or not doesn’t totally matter, if they get into a “combat” state, unfortunately that objective fails.
I wish it were different though. Completely rules off any public long range assassinations with it.
No, I moved away immediately. Plus, the split second I killed the target I ruined the “no combat” status. I think @Heisenberg explained it very well, I wasn’t aware of the problem of guards witnessing the deed.
Yes, I replayed the mission, and the finally I selected “Hide and seek” and passed it. This “no combat” was impossible to lure the target away from the public.