I just put some cheese sandwiches into my microwave oven, and I can see a small layer of fluid on the surface of the cheese (trappista). What's this and why I can't see it before heating cheese up?
I assume this is fat, but I'm not sure.
It is almost certainly either fat, water, or a combination of the two which was squeezed out of the cheese as a result of the heating.
The heating would cause the proteins in the cheese to further tighten, squeezing out the liquid, which is why you may not see it until heating the cheese.