I got one or two a long time ago. I remember getting one on Venus, but I'm not sure about the others I have killed tons of enemies on all the planets when farming exotic material or completing bounties, but I haven't had any drop recently. With that being said, maybe they only drop randomly when completing patrol missions on a planet. This is about the only thing that I do differently, Maybe, in order to get the random drop, you have to complete a certain number of patrol missions, so as to activate the random drop. Who knows, maybe it won't drop if you don't do patrol missions?