It's the second mystery quest from Jones. If you have those who haven't completed all the quests go and read all of the quests in the guide, I'm pretty sure that one will be the one they don't remember doing. Go to Jones, accept his daily quest, and do the next one he offers you.

It's been brought up numerous times to make it so uncompleted quests show up before dailies and such. It seems this issue isn't a high priority at the moment.