Great question — answering as the UphillTech team:
The only way to know for sure if you have an active leak is to cut open the ceiling and take a look at the pipes and drain. This is not a difficult task, and then you'll know for sure. If there's high humidity in your area, on warm days the cold water running through the pipes will make the air around the pipes condensate, creating water droplets around the pipes, some of which might be dropping on the ceiling below. There's nothing a plumber can do about that. On the other hand, it's still possible that you are just seeing the previous stain bleeding through. Use an oil-based stain-blocker paint primer and that should take care of that.
Full answer by Cheery: https://diy.stackexchange.com/a/304761 (CC BY-SA 4.0).
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