On Sep 23, 4:57 pm, dpb wrote:
The water stain indicates a leak.

Whether it's actually the drips hitting the ceiling or inside the drain
is of little consequence; the leak needs fixing.

The thing is the dark area on the ceiling is so small and slow-growing
that I'm not even sure it's a leak. It's maybe 6 inches long and a
quarter inch wide, along the edge where the ceiling meets the molding.

As another poster said, investigate for dampness in the plaster and the
ceiling area. If it is drain rather than supply, not using the suspect
sink for a while should allow it to dry completely whereas a supply line
leak would continue.

How long should I avoid using the sink to wait for it to dry?

I think the drain pipes are PVC, but if they're copper, does that mean
a dripping noise definitely indicates a leak?

Jimmy