Seen 3 drop for my groups in the past week. 2xDreadcutter Heroic (1xCaster,1xHealer) 1xWurmbone End Heroic (1xHealer)
Allegedly they do drop in solo instances as well, personally never seen one in solo zones but only in group zones, followed by half a day of drama cause people can't resist ninja looting them.
They do drop in the advanced solo instances but I wouldn't guess they have a higher chance. I have had one drop in who knows how many runs.