Usually when you kill someone, a red skull briefly appears on the screen along with the name of the player you killed or assisted in killing. However, with Mercy, this red skull also sometimes appears even when nobody is killed.
What do these "nameless" red skulls mean? My only guess is that you get them if you damange boost a certain amount of damage, say 200 hp worth of damage, so it kinda counts like a "kill". I would even say that damage to barriers counts too, as I got those red skull when I damage boosted a Bastion firing at a barrier.
So what are they really?