Your original post failed to mention that the stain has been there for
a couple years and hasn't changed. I think everyone thought this was
recent and an ongoing problem, which is why there were suggestion to
cut open the ceiling. If it hasn;t leaked for 2 years, I don;t know
why you would even call a plumber.

If the drywall is ok other than being stained, I would put a couple
coats of stain killer on it, then repaint that area. Wait awhile, and
if no further leaking occurs, paint the whole ceiling.