A smoke test can help to find a vacuum leak but doesn't mean that there is no vacuum leak if you don't see smoke. Kinda a "negative option" thing. Since the crankcase ventilation is closed any part of the crankcase (including pan gasket and rear main seal) can leak air in. The intake gasket itself is rarely the source of the leak and there are lots of dynamics involved as to why only one cylinder triggers a code. Vacuum cap at the back of the intake and the brake booster would for example be good places to look for bank 2 but that doesn't rule anything else out either.

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1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)