I doubt this is your issue but a leaking waterpump o-ring can cause your coolant to be forced out of your overflow tank too. When the pump is running it can suck in air and force the coolant out. When the waterpump isn't running (ie: car is off) you'd have coolant coming out of the leak if this was indeed the issue. Like I said, doubt its your problem but I figured I'd just throw this out there for anyone else searching who may have a similar symptom.