The monsters/quest items are randomly distributed among the lowest levels of all the dungeons. So in each dungeon you can find none, one or more quest items/monsters.
For each dungeon, there is a separate key. (Note: a level x key unlocks all the dungeons up to and including level x -> so the level 3 key in act 1 also unlocks levels 1 and 2)
If you need help locating the keys:
SPOILER ALERT!
Act 1 BF key 2: in the barracks (nr 4 on
this map)
Act 1 BF key 3: samuel's chamber (nr 8 on
this map)
