On Wed, 10 Aug 2011 13:01:12 -0500, "Dave" wrote:

Have discovered that our kitchen sink drain appears to be leaking inside the
wall behind the cabinets


I only know what I know, which is steel/iron drains in the walls.
1.
A. A normal kitchen drain has a stub (usually 1 1/2") coming from a T
inside the wall.
That's the hole in the T facing the sink.
B. The top of the T has a vent pipe either going straight up through
the roof or attaching somewhere to another vent pipe that does.
C. The bottom of T has the drain pipe, and takes waste water to the
drains.

2. The horizontal pipe leaving the trap under the sink goes into the
stub attached to the T. (A)

All the ones I've seen slip in easily. You normally just dope them up
and slide them in the stub, which is a threaded pipe nipple.
Since the horizontal trap pipe should go into the nipple at least
about an inch for a good seal, it's most often that's a nearly exact
horizontal fit..
A nut with a rubber or nylon "compression" type washer is then
tightened up. That seals best with a near exact horizontal fit.
Since there's normally very little pressure it won't leak.
That nut should be outside the drywall and in the cabinet under the
sink.
Since you can't see it, its a half-assed installation.
Cut out the drywall so you can see it.
The plumber makes you pay for anything he does.
You'll know how it works when you see it.

A possible cause of the dampness/mold is the drain needs to be snaked
and cleaned. Clogging can create enough pressure to overcome a weak
seal.
Another possible cause is the water is leaking from elsewhere and
ending up behind the sink.

--Vic