Could be a number of things. Uppipe leaking, uppipe gasket leaking which is most likely the issue. Could also be either side exhaust manifold leaking at the cylinder head with broken bolts.
There is also a chance that between turbos is leaking on both the hot and cold side, best bet is to get under there and look for soot trails, even a small hint of it could be your issue. Then the next thing would be to start it up cold have someone rev to about 1000 rpm and feel for leaks around the turbos ect.