The kill-confirmation prompt whose absence tells Carl his hob-lobber silenced a gun without killing anyone.
When Carl hurls a full-strength Hob-Lobber at the enemy Heavy Pulse Gun, he sees a flash but receives no experience notification. The absence is diagnostic: nothing died. The gun stops firing anyway, so he cannot tell whether the crew was driven off, the weapon damaged, or the reprieve momentary — which is exactly why he follows up with a Troll Smoke Mantle before advancing.
It is one half of the interface's grim bookkeeping during the battle; the other is the Casualty Notification plink for every soldier Carl loses, which he ultimately has to mute.
The kill-tracking mechanic Carl reads to judge whether his grenade landed.