I fried fresh sliced button mushrooms in butter over low heat with no other ingredients. After a short while dark red areas started to appear on the sides of several slices.
I never noticed anything like it before and would be happy if someone had an explanation. Also I would like to know whether or not it is a sign of some sort of danger.
I tried to find information online but the results are mostly recipes and the only instance where someone describes the same, people's responses are not at all helpful (»maybe you cut your finger without noticing«, »maybe you used fly agaric«).