Now, for those which want a direct explanation
- Every player has a % chance to get some loot, if he does not he will be given gold instead. The loot will be added to your inventory directly (this does not currently work so you have to loot the boss). Now, you may or may not get loot on the boss, every player sees his own
- If you looted a boss (You and not the raid, you) you will not be able to obtain loot from that boss for that week, so its only one drop for a player per week
The boss or chest vanishing means that the ones who could loot it already looted it, that's all there is to it. Nobody stole your loot, you just weren't lucky