I turned in 32 of my rank up engrams with Nimbus in one sitting. I got 1. I was told that the gold patrols offer them at 50% probability. I haven't seen one drop in a week and I've run probably 10 of them in that time. They shadow nerfed the drop rates after all the first week grinders exploited those drops. Only explaination. Too many people got them in the first week so -blam!- everyone else.