Radiator leak...stopped for now at least
I recently found the coolant expansion tank empty (thanks to Dean's post), so I filled it to the appropriate level and thought all was good.
Later I noticed coolant leaking at the rear of the radiator where hoses are attached. After tightening all hose clamps I thought all was good now!
Again I noticed coolant dripping from the same area. It turns out the leak was coming from around a bolt near the top of a vertical row of bolts on the rear of the radiator (see photos). It looked like it was from the core and I was thinking of Barry's recent $5K repair. 
But on closer inspection those bolts look like they hold a tank onto the side of the radiator and there appears to be a gasket or seal at the joint. I tightened all of the bolts in that row (almost all of them would easily take about 1/4 turn). I ran the engine at operating temperature for 25 minutes without any evidence of leaks. Now we have driven about 200 miles to our current location on the Oregon coast and it is dry.
I am worried about this though because there is another row of bolts outboard and hidden by the charge air cooler. I guess I'll just keep a close eye on it.
Topic: Radiator leak...stopped for now at least