Well, the leak itself isn't a mystery - it's at the upper radiator hose outlet of the radiator. Today, after a long highway trip, I noticed my cooling fan was running with the engine off (probably unrelated), popped the hood, and saw the telltale dribble. Some had also managed to spray across the charcoal canister onto the block, where it had pooled under the t-stat housing. At first I thought the housing itself might be leaking, until I saw the spray signs.

What's really weird about this is that the radiator is less than 2 years old, and its predecessor (also less than 2) actually cracked underneath the same spot (between the fins and the plastic housing), presumably due to a high-pressure situation. I've since switched to the lower pressure cap. Also put in a brand-new upper rad hose recently - that puppy's on tight, and nothing should be leaking at this point.

I'm thinking headgasket, even though there's no sign of cross-contamination. Does anyone have any similar experience? And does anyone know of a test kit for detecting potential gasses in the antifreeze?

Many thanks!