98 Pontiac Trans Sport 3.4 w/coolant coming out the top of the overflow tank while engine is cold?!?

I've been losing about a half gallon or less of coolant a month, but haven't been sure where it's been going. Usually I've waited until the low coolant light comes on before I refill it. Then, for the first couple of weaks I can smell a hot coolant smell inside the van while driving, especially when I come to a stop it gets stronger, though it never overheats. The smell goes away, but then a couple of weeks later the low coolant light comes on. Also, the level in the overflow tank goes down too, but never empties. It seems to remain about half way to the full mark (from the bottom) steadily. This morning I refilled the radiator with the engine cold, then went out 2 or 3 hours later and found a very small puddle underneath. I followed the drips all the way back to the top of the overflow tank, where the cap is, which is like a rubber stopper. The radiator fluid was condensating on the bottom of the cap and traveling up and out of the tank. How is this happening? Freak incident?