the origin. Most of the time one can logically and professionally GUESS that it has to do with a stack or hole in the roof, but that does not have to be the case. I personally HATE repairing leaks in homes, because the leak might take three times out to find it and the homeowner does not understand that.
I find leaks most of the time by using a flash light and proceding to the attic and finding that stained water trail that water will leave. If the stack was re caulked and checked on, then you might have another problem.
Where is your AC unit located? is it in the attic? AC ducts also will leak from condensation.
Do some checking in your attic and report back. We probably can assist you further.