Supplied and Ready

This is a compilation of other comments, so credit goes to them.

You need to be in Warmode to see this chests.

Cache is visible on the minimap, marked with a chest with crossed swords behind it.

Caches are lootable once per incursion, up to 7 times per week (each day one, since they have a daily cooldown). They are not always up, so you might want to camp spawn point or use realm hop. Joining groups doing World Quests is easy way to get onto other shards. Chest despawns in a couple of minutes or after a certain ammount of players looted it (speculation).

Loot contains a random PvP blue ilvl 330, like random battleground rewards do, and conquest or honor points. Loot is shared with regular air drops and can be obtained only once per day. It is not, however, shared with bounties for assassins.

Coordinations of known locations (looks like it is 3 spawns per zone, most of them are personaly verified)

• 
  
• Tiragarde Sound:
  
/way Tiragarde Sound 74 44 Secret Cache
/way Tiragarde Sound 79 37 Secret Cache
/way Tiragarde Sound 83 36 Secret Cache

• 
  
• Drustvar:
  
/way Drustvar 33.8 49.3 Secret Cache
/way Drustvar 33.7 40.75 Secret Cache
/way Drustvar 29.6 41 Secret Cache
/way Drustvar 30 33 Secret Cache

• 
  
• Stormsong Valley:
  
/way Stormsong Valley 46.63 56.57 Secret Cache
/way Stormsong Valley 43.62 48.25 Secret Cache
/way Stormsong Valley 40 62 Secret Cache

• 
  
• Zuldazar:
  
/way Zuldazar:Zandalar 70.65 37.5 Secret Cache
/way Zuldazar:Zandalar 80 51 Secret Cache
/way Zuldazar:Zandalar 81 55 Secret Cache

• 
  
• Nazmir:
  
/way Nazmir 68.69 13.05 Secret Cache
/way Nazmir 60.9 9.67 Secret Cache
/way Nazmir 78 19 Secret Cache

• 
  
• Vol'dun:
  
/way Vol'dun 33.6 45.3 Secret Cache
/way Vol'dun 38.6 57 Secret Cache
/way Vol'dun 36 50 Secret Cache

• 
  

Edit: added/corrected some coordinations based on comments, not verified by myself though.

Maps showing chests:

Zone

Alliance

Horde

Drustvar

[object=316779]

[object=316784]

Nazmir

[object=316780]

[object=316786]

Stormsong Valley

[object=316778]

[object=316783]

Tiragarde Sound

[object=316776]

[object=316777]

Vol'dun

[object=316781]

[object=316785]

Zuldazar

[object=316782]

[object=316787]

Hunters out there if you use your Eagle Eye [spell=6197] there are locations where you can watch for each of the Secret Supply Caches.

For example, in Stormsong Valley I stood on the roof at 43.6, 55.4 in Deadwash, from that location I checked each spawn point until it spawned and I was able to loot.