From another thread:
In particular, there's a level 3 dungeon room that gives you bonus exp for your light, but the monster there is always face down.
I saw one mention of this in the rulebook but have not found where it officially tells you to place monsters face down. In particular, during the Setup, the rulebook explicitly tells you to place all monsters face up.
Can @AEG please swing by and explain when/where monsters get placed face down? Thanks.