Rad was low (1/3 gal) and the overflow tank filled up. No bubbles that I could see. The strange thing was watching the top hose suck flat as the motor cooled down. Are you thinking head gasket? Would it blow all of a sudden with no external leaks, no prior water consumption or water in the oil?
Just looked at it this morning and the bottom hose was also sucked flat. I removed the rad cap and there was a suction "whoosh". The thermostat has gotta be stuck shut, right?