Okay I'm stumped

[79BIG10]

I had been noticing that the coolant level in the radiator goes down to the same level after a few days of driving. I've added and added but again it goes back down well below where it should be (less than 3/4 full). I've replaced the heater core and still doing it. No leaks since the replacement and also have ran the engine to let the thermostat open up to get all the air out of the engine. It doesn't overheat at all just loses alot of coolant. I've checked the exhaust and nothing smelling sweet or any clouds of smoke. No water in the oil pan either.

Now I think that it's overflowing into the overflow bottle and not draining back into the radiator. Isn't it supposed to? I've changed the radiator cap and it's on 16lbs. rating. I see a couple drips from where I park and come out and it's leaking at the bottle but I check and it's right at the perfect level. Is there something I'm missing??