Well, at the moment that we found the mold, our water heater had a small leak. The damaged area was flooring and wall in an area which held the water heater, a laundry room with deep sink (the idiots we bought the house from had the washer water draining into the deep sink. So of course, every time someone tossed a dirty rag in the sink and it went unnoticed....FLOOD!) and a bathroom. The damage was extensive and it's likely they had a number of overflows.
What really frosts me about this is ANYONE who had half a clue should have been able to find the mold. We had TWO house inspectors (one we paid and one VA) and neither one saw the mold or even tried to close the bathroom door. The door frame was warped and the door did not close.
We knew very little about buying a house and hired people we thought would find problems we didn't know to look for.