Is it orange OAT coolant and have you got silicone hoses ? With heat soak your coolant and therefore hoses get hotter and more pressurised when you switch off.
I had something similar happen on my Elise when I swapped to silicone hoses. I have always suspected that the hose developed internal pin holes, (especially where a tee branch occurred and the two hoses forming the tee were bound with more tape on the outside) that leaked into the fabric re-enforcement and thence out of a couple of microscopic holes in the outer case and also sweated coolant out via the fabric at the ends of the hoses. This you could see by the crust on the hose ends.
There is much evidence that OAT coolant eats silicone hoses, hence my suspicions about the cause.
Just a thought
It is ordinary "Red" coolant mixed with 50% water. I also add a can of original Bars Leaks every time I change the coolant (at springtime EACH year). No silicone hoses. I think all the hoses are the originals from 2005.
