Hi guys, thanks for all your suggestions,
BLIMP: that forward outflow valve is only opened by two things, one, either aircon pack selected to high, part of the increased ventilation for contaminated air removal, (and then only partially open) and two, with less than 2.5 psi cabin differential,
good thinking, but not the answer, I think.
B73567..., some B737s have two recirc fans, some one, and some none!!
All good, well thought out answers , but I don't think we've found the right one yet.
Cheers