I have a slow leak in my living room ceiling. It is not near the kitchen or downstairs bathroom. Is is not under the upstairs bathroom. It is on the bottom floor of a 2-story house so it's not the roof. The leak is not near an eave or gutter an it's hardly rained in the past few days anyway. However, the leak is almost directly under a radiator (the old fashioned steam heat kind) in an upstairs bedroom. There is no wetness up there but I can only conclude that it's a leak from a pipe that goes through the ceiling to that radiator. Is that likely? Any other thoughts? I've called a plumber but am trying to diagnose beforehand. This portion of the house goes back to the 19th century so who knows what's happening inside the floors and walls!

That scenario is entirely possible, you can do you plumber a favor and remove the drywall/ plaster ceiling where the stain is and expose the piping… although I will say if the steam pipe is leaking in the 1st floor ceiling the 2nd floor bedroom floor should have some kind of dampness or buckling of flooring, have you removed the escutcheon plate around the radiator piping through the floor in the bedroom? Or maybe take a small mirror around the suspect rad piping and see if the mirror fogs up

Thanks! I did remove the ceiling portion and that was right--an old and damaged joint was leaking from the boiler. They turned it and off and the plumbers are coming back to fix it.

Nice investigating