I was asked by a mod to write a post concerning the smoke/gas that Harvesters, and Tanks produce when they are killed. I am almost level 50 now, and I find myself playing with lots of different diverse groups of players. I frequently find newer players that would like to grind on enemies, and the enemy with the highest rewards (Loot/EXP) is the Harvester, (Though some tanks also have this aura.) When a Harvester is killed it produces an invisible aura of gas that harms the player upon contact. I was informed this effect is intended to have a visual appearance like the gas that can be found in the open world; or gas produced by large unit AoE. To newer players this bug seems to be cause for confusion, as well as frustration.
TLDR: Harvesters and some Tanks excrete a toxic gas when killed, gas aura cannot be seen. (I have been told this was intended to be visible)