Hi...I've got a water stain (I'm assuming it's water) which has been
growing on the ceiling of the main floor. Above the spot where the
stain originally began (it's grown longer now) lines up exactly with a
sink in the upstairs bathroom. I don't know if that is a coincidence.

I've let the water run in that sink for a long time and I don't see any
obvious leaks.

The plumber's I spoke to don't seem to have a good idea how to go about
this kind of investigation. I certainly don't want to cut a hole in
the ceiling as the first thing I try (as a couple plumber's suggested
to me).

Can anyone suggest how one can go about locating this source of the
water? In the bathroom in question is also a hot tub, toilet, shower,
and another sink.