Water follows the building before it falls
Rain entering near a pipe or flashing joint can run along the underside of the deck. A framing member can carry it farther before the water drips. Insulation can hold moisture and spread the visible effect. The room where the stain appears does not always identify the roof detail above it.
The path can also change with wind and the amount of rain. A connection that sheds water in one direction may admit wind-driven rain from another. That pattern is useful information when you already noticed it, but it does not establish the exact opening.
