Today I notice the needle was just below the halfway point, normally its in the middle of the gauge. Once I got back home I noticed the overflow tank was empty. I couldn’t wait for the car to cool so I grabbed a towel and opened the radiator cap, there was no pressure or steam and there was fluid all the way to the top. The car had been driven 10 to 15 miles and temp is in mid 80s today. Any ideas?