@Snarf Zagyg I hate to be a stickler but I’m not sure you can move HM that way. I think it always has to be on a target. As soon as that target is reduced to 0, you have to move it to a target you can see.
So, when you were fighting outside, your target either:
1. Never dropped to 0 or;
2. You moved it to a target you can see at that moment (which wasn’t the current zombie)
I’m open to discuss and I may be misinterpreting.
Here's the text of the spell (my emphasis is in underlines)-
You magically mark one creature you can see within range as your quarry.
Until the spell ends, you deal an extra 1d6 Force damage to the target whenever you hit it with an attack roll. You also have Advantage on any Wisdom (Perception or Survival) check you make to find it.
If the target drops to 0 Hit Points before this spell ends,
you can take a Bonus Action to move the mark to a new creature you can see within range.
Using a Higher-Level Spell Slot. Your Concentration can last longer with a spell slot of level 3–4 (up to 8 hours)
or 5+ (up to 24 hours).
So the way I interpret this is as follows:
Hunter's mark operated "until the spell ends" on the target.
If the target drop to 0 hit points before the spell ends, you can then take a bonus action to move the mark.
However, there is no "trigger" on the bonus action. It doesn't say immediately, or before your next turn, etc.
In other words, it's a concentration spell, and "before this spell ends, you can take a Bonus Action ..."
And this is important for two reasons- first, a lot of the time the target might die due to your attack and after you've taken your own, different, bonus action. So it can't be on your own turn. Second, the target might die due to someone else's attack after your attack. That means that (IMO) you are simply maintaining concentration (which means you can't cast other concentration spells) and have to take a bonus action in the future.
I think that this reading (again, IMO) is further confirmed by the upcasting options that allow for it to remain active for a long time?
That said, this is how I read it. I accept whatever the ruling is. (And given Caer is melee focused, it just means more concentration checks even if it is active)