Something is going on with this ceiling. The homeowner noticed a growing stain on the ceiling that seemed to get worse every time it rained. This corner of the ceiling is under a low area of the roof and located directly under a roof valley (junction of two sloping areas of a roof). This stain is a good indication that the roof is leaking.
There was a previous leak in this same area, you can see the square area where a previous repair has been completed. Obviously the leak wasn't repaired adequately or a new leak has formed in the same area.
Common areas of roof leaks are roof valleys, junctions where a vertical surface abuts the roof or penetrations from pipes through the roof. The homeowners had a roofing company inspect the roof and they found an improperly installed valley flashing. The roofers repaired the flashing and made the roof water tight again.
Once the leak has been repaired, it's time to open up the ceiling and assess the damage. The drywall needs to be cut from this section of the ceiling and any wet insulation needs to be removed. Inspect the attic area and try to find all the remaining water on the joists and wooden components. The repair area needs to be left open to dry before completing any repairs.
Brian Jovag, owner of Jovag Home Inspection.