Leaking Steam Radiator Pipe?
Water can run for some distance inside a ceiling before finding a path to visibility, so the leak might not be where you think it is. Check all the radiators above the first floor for visible leaks or wetness.

The leak could be very small and intermittent, for example dripping only when the system first comes on and warms up, then drying off before you see it on the pipe. Try this -- wrap all suspect pipes with clean paper towels or similar tissue and periodically check for staining.

Another possibility is that the stain isn't from the heating system at all, but is the result of a bog standard domestic spill near the radiator, and some of the liquid found its way through the pipe hole in the floor before it was cleaned up.

Getting rid of the stain is complicated by the fact that the puddle above the ceiling is larger and wetter than the visible stain. You want to make sure it's completely dry before applying stain remover and paint.