No, CGs do NOT drop from any location in the game. You can only get them from Fortuna and the Shop.
The rumor going around about CGs in Clouds is BS. Players lie because theyre buttholes. You can only attack a Cloud by entering the coord in the Muster Point Attack screen. Doing so simulates an attack on a Lvl 10 Plain with 5000 Gold and 5000 Food. Those circulating the rumor of CGs being hidden in them are just trying to waste other peoples time.
Clouds are simply Dev test sites blocked off in each realm for troubleshooting and testing new features.