All the boxes we've seen so far required Lockpicking 75 to unlock regardless of the level required to actually open the box. Both the 75 and 80 rogues were able to unlock them. I was also able to unlock them on my level 70 jewelcrafter using the legion
Jeweled Lockpick, although that's obviously not intended. Our initial thought was to try a quality over quantity approach (elites, rares, delves, dungeons), but then we ended up settling on the exact opposite due to lackluster results. As far as I can tell, every humanoid NPC has or should have a worthwhile chance to have a box in their pockets. If you have more luck with the stronger foes, we'd love to hear about it.