You look at the fiction and you make a call.
If you are fighting some Goblins together with your man-at-arms and they take the brunt of their counter attack, they'll be bleeding from some stab-wounds, but they'll probably live. If you are fighting a Giant together with your man-at-arms and they take the brunt of their counter attack, your man-at-arms will be flat like a pancake.
Same with the burglar. If the trap is a poisoned pin-prick, he won't take any damage but without immediate healing he'll probably fall unconscious and/or die from the poison. If it's a fireball trap, he's likely on fire and needs to be doused in addition to having his burns bound up. If it's a blade trap, he's likely to lose a hand.
Remember that the game itself refers to hirelings as sorry fools on a foolhardy quest. They aren't adventurers, but wannabes. Their exact HP isn't important; what matters is that they give the GM a chance to show that the world is a dangerous place, and that most adventurers end up like corpses in dungeons. Use them for that purpose and don't sweat the details too much.