Looking back at how you describe it (and looking at the floor plan), I have a couple ideas.
With the damage along the wall near the fridge and dinette makes me think either the fridge roof vent is leaking, or the fridge condensation drain is draining on the floor behind the fridge. My top suspected culprit,however, is something in the bathroom, probably the shower.
If it is leaking because of plumbing, or even because someone didn't pay attention to the shower curtain while showering, it could allow water to migrate along the floor to the side and the back walls.
The roof vent leak would be a reason to run as it could do some serious damage. The other two, might not be a huge deal as long as they have been corrected before anything major happened outside a little swollen luan. It might be worth having it inspected by a pro to determine which it is.
