Coolant Leak

Guys, I have a real puzzling problem. The last two winters, my wife's 2015 ST Limited Spyder has leaked coolant out of the overflow tube. During riding season its works fine - no leaking. I've checked everywhere for other leaks and don't see any. I put a pan under it so I can tell exactly where it is coming from - the clear plastic overflow tube that comes down from the back side of the tank and out the bottom of the machine. This just makes no sense to me. I would think the summer time when the engine heats up is when it would come out the overflow tube and not in the cold Illinois winter weather. However, it sits in my garage in the winter which never gets below freezing. I'm very confused. Can you help?