Hobgoblins' eye changes based on whether or not it can use Thermal Barrier. I've killed well over 10,000 Hobgoblins and yet not once noticed this small detail until I was just mucking about with the HUD off.
[spoiler]Left: On cooldown
Right: Active[/spoiler]
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Did you know...Shooting a hobgoblin with a sidearm doesn't allow them to use the thermal barrier at all. Some people still don't know this