Could be the connection part of the various hoses or the o-ring within has loosened out. Your radiator test should be done when the car engine has cooled down. When the car is heated up, chances are you can't see any leakage as all the metal and rubber part of the cooling system and hoses would expand, thus tightening up all the connection and 'loose' part. However when the car cool down over a couple of hours, the said parts would also shrink and expose the loose part. That is why some workshops would use a big fan to hasten the cooling process of the engine area to check for the leak. Some workshops would just ask you to leave the car overnight for them to check for the leakage spot the next morning. Although the shrinkage would be minute and of course not visible to the naked eye, it would be 'big' enough for the water/coolant to seep thru. Hence the minor leakage. Speaking from my own experience, I can also see a small spot of water just beneath the affected hoses. However it doesn't leak everyday, sometimes can see the minor leakage, sometimes can't notice any although the car was left for a few days when I go overseas. So far the small leakage cause me to top up one glass of water every month.
