If a hero opens a chest containing a monster (ex : Hidden Zombie), does the zombie appear immediately, or at the end of the hero's turn ?Immediately.
If the monster appears immediately and is alone, it would give an automatic XP to the hero. Yes (if the monster is a zombie or a giant rat). This is certain kind of a reward. But if the hero wants to attack this monster, he can be delayed, as he has to end his moving phase on a space adjacent to the monster.