June 24, 2016: Had to have the XK8 towed AGAIN . . .
I was on my way to the airport in Atlanta to pick up a friend who was visiting when I saw a "Low Coolant Level" warning message on my instrument cluster. I checked the oil, tires, and coolant before I left the house, and all were full. The temperature gauge was right where it should be when the light came on. But roughly 2 miles later, it shot up into the red. A "Restricted Performance" message now joined the "Low Coolant Level warning. So something happened.
The outlet from the thermostat housing where the hose belongs can be seen in the photo above. But enough pressure had built up to blow it loose despite having two clamps on the hose. I believe the water pump impeller fins, (which are plastic - yeah, that's a great idea), disintegrated. With the impeller fins gone, there is nothing to circulate the coolant. When it got hot enough and built up sufficient pressure, it blew the hose off of the outlet. I'll find out for sure when I get the car home and start taking things apart.