That line should be connected to the overflow tank in the fender. The expansion tank holds pressure and siphons back and fourth to the overflow tank. Thats why the expansion tank should never be topppped off hot or cold or youll always end up with water on the ground from the overflow tank. When it gets hot and relieves some pressure (which is normal) it will sipphon back as it cools down. Just re run the line and I am sure youll be fine.

Again if its that hot outside your system is working like a champp.