button mushrooms turned red while frying in butter 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«).
According to this article, button mushrooms may turn a pink color if bruised while being stored or handled. This sounds very much like what has happened to you, as you've stated that the mushrooms were old and not stored properly. This is not poisonous or bad to eat by any means, but I understand that one would not want to take the risk, especially when it comes to a fungi such as mushrooms.

For more information on storage, see the article provided above. A short excerpt on storage taken from that source:


  Storage - Agaricus bisporus will last longer in your refrigerator than
  many other mushrooms, usually around a week. It's better to store them
  in a paper bag rather than plastic. They'll last even longer if you
  take the time to put them in paper!