Solution for a Small Leak
Some roof leaks are tough to locate. Sometimes the water shows up at a ceiling spot distant from the leak. If your ceiling has a plastic vapor barrier between the drywall and the attic insulation, push the insulation aside and look for flow stains on the plastic. Often water runs to openings in the vapor barrier, such as at ceiling light fixtures.
If you can't see any telltale flow marks, and since the stain is fairly small, look at the underside of the roof for 'shiners.' A shiner is a nail that missed the framing member, in this case when the carpenter nailed the roof sheathing to the rafters. Moisture that escapes into the cold attic from the rooms below often condenses on cold nails. Sometimes you can spot this if you climb up into your attic on a cold night. The nails will look white because they're frosted. When the attic heats up a bit during the day, the frost melts and drips, then the nails frost up at night again and so on. The solution is to simply clip the nail with a side-cutting pliers.