When the quest instructs you in the setup section to place monsters on the map in specific areas do you place these monsters regardless of where the heroes enter the map? We have been playing that until the heroes open a door to a section that would contain those monsters they are not placed on the map, but I cannot find anything in the rules that makes me think that's right.
Yeah that's not right. There are certain exceptions where monsters aren't placed during setup, but the rules will very explicitly say so.
Not only should you place the monsters before the heroes open the door, the monsters can open the door and attack the heroes first.