Spoiled Goods

Those boxes spawn only in warmode in FFA pvp parts of each zone, you can see icon on map similar to normal air drop
Dropped 500 honor and [item=213778]

UPDATE: seems now counts for achievements regardless if you open it first or not
UPDATE 2: it spawns once every 6h in one of zones (00 ST and onwards), always same place in every zone, important note is than when Hallowfall is chosen - it's bugged right now and doesn't spawn at all, also Azhkahet one doesn't count for 10 spoils achievement, and is not visible on main zone map as it spawns in city of threads subzone

Finished this today and want to piggyback on previous comments, add some input, and clarify a few things here.

1. Unbound Spoils spawn 4 times a day, once during every 6-hr intervals, starting at daily quest reset. (E.g. For NA, 1st spawn during 7am~1pm, 2nd spawn during 1pm~7pm, 3rd spawn during 7pm~1am, 4th spawn during 1am~7am)
Find those locations here.

2. Contrary to some previous comments suggested, the chest does NOT neccessarily spawn at 7 o'clock or 1 o'clock sharp, but rather randomly during the 6 hour intervals stated above. It can spawn on the hour, but it may not.
This makes farming unbound spoils very time-consuming.
Over the past 48 hours, I have seen chests that spawn on reset, ~45 mins in, ~1.6 hrs in, ~2.5 hrs in, ~3.6hrs in, ~4.4 hrs in.
Nonetheless, they never bugged out and not spawn.
(yes, I slept very little to figure this out, and I'm dozing off as I'm typing...)

I think the spawn timer confusion stems from the fact that some commenters are lucky enough to see multiple on-the-hour spawns, and its easier to believe this is a fixed-time spawn. However, the timer is actually random, hence the conflicting reports of random sightings of chests.

3. The spawn location is tied to active Shadowlands PVP quest zones, which means the spawn location is predictable. (Big thanks to clancyBY for this amazing discovery, absolutely goated)
Ardenweald - Isle of Dorn
Maldraxxus - Hallowfall
Revendreth - Azj-Kahet
Bastion - The Ringing Deeps

Given this linked relationship, important inferences can be made.

a) Check which Shadowlands PVP quest is active now, you can go camp out the corresponding map, the chest might not have spawned yet.

b) At 7 o'clock and 1o'clock, check which Shadowlands PVP quest is up next, and you can use dungeon teleports to quickly move to target location, in case its an immediate spawn (E.g. Ara-kara for Azj-Kahet, Priory for Hallowfall, Darkflame Cleft for Ringing Deeps).

c) Unbound spoils will never spawn in the same zone consecutively, just like Shadowlands PVP quest will always change zones when one expires.

d) The Shadowlands PVP quest zones make a full rotation every 24 hrs, so does unbound spoils spawn locations.
Therefore whichever zone it is active now, it will be greyed out for the rest of the day. (A day's start defined by daily quest reset)
Given the first 3 spawn location of the day, the last spawn location of the day is always known beforehand.

Happy hunting.

So, the chest does not appear in a random location, but is tied to the Shadowlands pvp locale
Ardenwald - Dorne
Maldraxxus - Hallowfall
Revendreth - AzKahet
Bastion - Echoing Depths