My guess is that when your turbo is sucking up less air than is being rammed into the intake, the excess goes into the engine bay. Do you plan to plug that and see what happens?
Hmm, why does this look familiar?
Right after the intake in my 1.6HDi there is a small box.
The manual calls it an 'Air Filter Box', which is funny, because there is no filter in there (this is where the 2.0HDi's filter is located, though), the air filter is behind the engine.
In fact, it's not really a box. When you open the cover you can see that it is only one wall, the rest is open to the engine bay.
Why? My guess it that when the variable-pitch turbo suddenly needs more air than the intake nozzle can easily give, then the rest is pulled from the engine bay. During normal operation air is rammed through the nozzle into the air box, so that most of what is fed to the turbo is fresh, not engine, air. Also, this air box may help to reduce the amount of dust and water droplets that continue on to the air filter proper.
I am thinking about totally
re-doing my pre-turbo intake with something more straight forward, sealed up, and thermally insulated.