There is a leak somewhere, because when looking in the radiator with car running, it looks like rapids. Coolant is swishing around. Now there is no white smoke and nothing in the oil. No puddles on the ground. I believe it is leaking through the rad or water pump because the level dropped some after it was filled and sat for a little bit without starting.

 

the coolant is likely evaporating before it can drip to the ground. the best way to find the leak is with a pressure tester with the engine cold. or look for coolant residue on the engine.