There is one very tricky possible location for such a leak, at least in the
2,3L Turbo in which  I have experienced the same problem.
There is wide rubber hose which goes back from the valve cover to the
intake. It has got a coaxial steel hose to circulate water through the
trottle body. The entire assembly crosses the cylinderhead and is fixed with
a bracket just below the jack of the DI cassette.  This bracket had worked
through the rubber hose but the steel hose was keeping it in place and on
the first view you wouldnt have expected a leakage.
The only indication I found was a certain amount of oil on top of the clutch
housing.
After I had fixed the leak the engine idling was very smoth and stabe at 900
RPM.