Will a crop ever just be totally empty or will it almost always have a little in it if the bird is seperated? I know at night it is supposed to go down because everything is digesting but will it be totally flat?
Yes, the crop should be empty in the morning - before the bird gets off the roost. Once off the roost, they tend to start gobbling everything up and the crop fills back up.
I have a hen, who the previous owners did crop surgery on for a sour crop. Her crop is distended and enlarged when she really is in a greedy phase it looks like a baseball swinging from her neck. Her crop is empty in the morning but not flat due to the stretched skin.