Some leaks only happen on the move, or when up to full heat. Try the hoses around the stat. Then the rad hoses top and bottom. Could be a small split in the reservoir, or even a knackered cap that lets out the water under high pressure when running. Keep an eye on it, have a look after topping up and driving a few miles. My car was losing a touch of fluid every week for ages, couldn't find the leak, then the bottom rad hose blew spectacularly. New one on and now no leak, must have been weeping out.
I would add that the coolant needs all removing and flushing and filling again with one colour. Doesn't matter which though. The two chemistries involved aren't compatible and cause sludge to form from the additives. Blocks waterways, overheats engines and kills.