I would do some caulking and eyeballing while this detective work goes on. I would get a ladder and apply some 35 year silicon/latex paintable caulking all around the exterior windows. I would look closely at the intersection of the bay window casing and the exterior wall of the home. Caulk would need to be there along with flashing. When this is done, then do the attic look see to find out if the leak is roof borne.