All the hose clamps tight? Hard to say without seeing it. One of the Trucks at work used to randomly loose coolant all looked fine but when you squeezed the lower hose it leaked like a sponge. Maybe your hoses are old? Check all joints and clamps physically with a spanner/screwdriver. Also check that the radiator cap is tightened all the way up to a sort of second click, as I lost a bit of coolant once cap wasn't screwed all the way round after checking the radiator level. Failing that perhaps a bike shop could pressure test the system for you?