It's the Operatives Cortex-Bomb.
I'm sorry, it is not. The Cortex-bomb has it's own (very small) icon of it's own.
For clarification, the symbol I am trying to identify appears on the same line, after the weapon used to make the kill, just like the Headshot symbol does. It seems to appear at random times, I've not been able to figure out the circumstance that creates it (hence, I don't know what it is!).
So the obituary would look like:
Mandrake (Rhett) [exploding melon] Victim's Name