Couldn't it also be the a/c condensate? If it is clean water and is happening even without rain that is where I would look. If the drain gets pluged the coil has no where to drain but the floor board. Just a thought.

Also when it 'mists' up is it just water or does it smell like anti-freeze. If things smell 'sweet' it is most likely antifreeze. Your heater core could be going bad or leaking....