MOLD problem in a guest room what should I do?

It has been smelling mildewy for months and we finally found our problem. we had the walls and carpet completely torn out to find it and it ended up being behind a moisture barrier or some sort of wrap that goes up about 12 inches from the floor behind the framing and insulation. But there is no sign of actual water damage....what could have caused this? The smell was so strong! We had the room fumigated to kill the mold and all the drywall and carpet taken out. Now all that is left are the interior walls and the framing. You can see where the mold was but it is supposedly dead now. what should be my next step? should that be removed? should we just put up new drywall and new carpet? What could have been the source of the mold? I am at my wits end...been dealing with this for months..maybe even a year now.