What counts as a noticed kill?

I just completed the Silent Assassin, Suit Only challenge in Colombia, and noticed that in my final score I didn't get the "No Noticed Kills" bonus. The only kill I made which could have been noticed was Andrea, who I killed via lethal poison, after which I immediately exited the mission (I was standing right next to an exit). An NPC definitely noticed her died, but I was under the impression poison kills didn't count as noticed kill, but it seems I was wrong.

Now I'm rather curious which kinds of kills actually contribute to that no noticed kills bonus.

Are there any kills which can be noticed by NPC, but won't be considered a noticed kill? In other words, exactly what's considered a noticed kill for the purposes of the "No Noticed Kills" bonus?

It is a bug/feature.

When you kill someone and it was unnoticed, you actually need to wait for the "unnoticed kill" to pop-up for it to count.

For safety, when you kill your mark, wait 3 seconds for it to show on screen, then exit.

The only exception is accidental kills.