I think it’s a combination of several things. As mentioned before, distilled water should be use to keep the mineral content removed from the coolant system. Dissimilar metal’s, aluminum thermostat housing, cast iron manifold and a metal thermostat sandwiched between the two. The gasket is probably saturated causing moisture to combine with metals and oxygen causing that corrosion. I have seen it on my car , especially when it’s not run for several months….. I don’t use distilled water either.
I’m thinking that the engine temperature helps keep the gasket from becoming saturated and if not run for a while, it gets saturated and wicks…. Just my opinion, not necessarily a “leak”