I believe this happens because players are in different "phases" or versions of the area depending on what quest objectives you've completed there. You may approach an area seeing it as it is in a different "phase," but when you get there it switches over to the "phase" that corresponds to your actual quest progress.
Taleof2Cities wrote: »I believe this happens because players are in different "phases" or versions of the area depending on what quest objectives you've completed there. You may approach an area seeing it as it is in a different "phase," but when you get there it switches over to the "phase" that corresponds to your actual quest progress.
You're on the right track, @Ilsabet, but the chest doesn't have to be quest-related for it to disappear. Another player can simply be in another phase of the zone and pick or open the chest ... causing it to disappear in your phase.