You can see that it is water. The red color indicates that something has colored the water on its way. If water comes through closed drywall, it is quite a lot of water. Be sure to check very carefully. If necessary, open it up in several places again and inspect thoroughly. Don’t downplay it. Observe carefully under which climatic conditions the stain changes. Because of such fun, we are getting a completely new roof and insulation in the fall. For us, it was a faulty membrane and 2 years later the insulation under the entire roof area is wet. This is just to give an idea of the scale of the damage water can cause under a roof.