speculation had been that it's for the triple-locked Arc chests in each of the 3 areas in the dreamlands, but that never worked for me. (plus those things are so buggy, I never even managed to complete the Rheasilvia one anyway) or that it could be related to the summonable bosses / saboteur chest / worm chest etc. - nope. just killed off a random high value target that was wandering near the spawn point in Divalian Mists, and that somehow completed it for me...? ok, sure, thanks bungie?