Quest reward

DaMore

Hello everybody, 
Can someone tell me what the small chests with the question marks under it means? 

When accepting a quest you can see the rewards you get for finishing it. Most of the quests have one of these chests as reward, but I've never get something out of it. Whats the point of this? 

Nina

I believe it is a random item or someone joining you, like the Drong questline