Can I use a reaction card in response to Minion being used for +2 coins?
Yes, you can reveal Secret Chamber in response to Minion no matter what.

Secret Chamber is a Reaction that can be played in response to Attack cards, and Minion is an Attack card.  The fact that the person played the card in a way that wasn't "attacky" doesn't change the fact that it is an Attack card.

The same goes for Pirate Ship when using its +$ option. Yes, this makes Secret Chamber an amazing counter against Pirate Ship.