Water damage in a ceiling
A ceiling stain is the end of the story, not the beginning. Water travels along joists and pipes before it finds a spot to come through, so the mark on the plaster is rarely under the failure.
What the shape tells you
A stain with a hard brown edge and a dry centre generally means the leak has stopped and the assembly has dried. A soft, spreading edge that is darker in the middle means it is still active. A stain that reappears after being painted over means the source was never found.
In a multi-family building the source may be two floors up. Water follows the path of least resistance through a wall cavity, then runs along the top of a ceiling until something interrupts it — a light box, a seam, a nail.
When it stops being a stain and becomes a hazard
Plaster and drywall hold a surprising amount of water before they fail, and when they fail they fail all at once. A ceiling that is sagging, bulging or weeping should be treated as a structural issue, not a cosmetic one. Stay out from under it and keep the room clear.
If the wet area is near a light fixture, a ceiling fan or anything else electrical, shut the circuit off at the panel before anyone goes near it.
Why it matters in older Maryland housing
Homes across the Baltimore rowhouse belt and the historic sections of Towson commonly have a century of accumulated mechanical layers running through the same joist bays: original radiator lines, later domestic hot water, later still a central air condensate line, and an ice-maker supply added last. Any one of them can drip for months into a plaster ceiling before it shows on the surface.