[EFFx]

Following shadow's advice will definitely help. I have a hacky way to detect players inside the smoke to block the "enemy: X" hud message. Basically I did what he said:

Check smokes around you. Get it's origin point. Check if you are closer enough (guess the radius is 115.0, you can make it up to 150-180 maybe), which will define whether you are inside of the smoke. This will be the first solution to detect kills inside the smoke (maybe both players inside the smoke, or even killing players in the distance, while YOU are inside the smoke).

You can use the same way to detect whether the victim is inside the smoke, so even if you are not inside the smoke, but the victim is, there's another solution to detect a kill through smoke as well.
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Last solution we'll need to find out is to how to detect a kill when both players are far away from the smoke, but the aim was through the smoke still.
Maybe making a traceline each 10-15 units, checking if there's any smoke around the origin of the trace?