Never a dull day in vanagon land, even when they are just sitting there.

I go to take out the trash, and discover that my 85, which has been
sitting un-driven for 3 days, is leaking coolant out of the very top
of the overflow tank.   It's never done this before.  I'm parking in
a new location, which leaves the back left of the van downhill
(perhaps 6 inches lower than the front) and in the sunshine.    So my
guess is that the thermal cycles of day/night, coupled with a small
leak somewhere, are letting air in the system?   Anyone have clue
where to look?  It has no leaks when running otherwise.