I don't feel that automatically attacking something that the player cannot see makes sense - in order to engage something in combat, doesn't one first have to know that there is something there to fight? I certainly cannot reconcile in my mind the image of an adventurer flailing his/her weapon about randomly with every step, hoping to hit an undetected monster by accident. In NetHack, attempting to move onto a square containing an undetected monster (and not marked by the game as suspected of containing a monster) prints an unseen monster detection message, reveals the monster if the player would be able to see a non-hiding version of the monster, and otherwise wastes the turn - perhaps the player bumped into the monster. I think this is the ideal way to handle unseen monsters, whether they be invisible, in a dark room, etc., and regardless of whether they are hostile or not. This also makes the monster hostile in NetHack, which does not make so much sense to me, either gameplay- or flavor-wise.