I had the same problem several times in my 528 (E39). I traced it to the carpets in front of the passenger seat being slightly damp (Don't know how. Could have been water spilt, could have been window down slightly when it rained overnight).
Anyway. It stunk like bad cheese, foot odor etc all put together.

Running the heater on max heat, max fan and blowing on the foot well sorted it in a couple of days of commuting...