Dave wrote:
. . . . . Have emptied the space below the sink and cut away a
portion of the back panel to reveal a moldy expanse of sheetrock
centering around the passage of the drain. Can anyone give me an
idea as to what might lie behind the sheetrock, and what I might do
about it.

P.S. Since you already cut out a portion of the back panel of the cabinet,
it doesn't seem like it would be much of a big deal to also cut out some of
the sheetrock and see what is going on back there. Maybe you'll be able to
see what's leaking and it will give the plumber an idea of what's going on.