Why do random red skulls sometimes appear for Mercy?

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?

Solution 1:

Those are assists.

When an ally gets an elimination and you contribute to the enemy's death through indirect means, such as weakening the enemy or buffing your allies, you score an assist. Assists are shown in the kill feed as small portraits next to the killer's, and are displayed on the assister's screen as red skulls and a small popup with the name of the enemy you helped kill.

The "nameless" red skulls you're experiencing happen when you assist a teammate in destroying something that is not an enemy player, such a shield or a turret. These actions display the same red skull and used to count as full assists, but don't anymore.