Okay, anybody got any hints on an easy way to determine where Flow is losing mass?

I've got a furnace process modeled.  Long muffle furnace so I put a 1atm pressure BC on the open ends of a segment of it.  Process gas is injected and exhausted (both defined as mass flow rates) in the center of this segment.  I get vortex warnings routinely so I tried switching one open end to a defined flow out (50% of (injected - exhausted)).  I left the 2nd side as a pressure just in case....

Well, when the pressure end still has vortex warnings (oh well, I'll live with a bit of resolution error there).  But when I query the Surface Parameters that end is nowhere near the other end for mass flow rate.  Somewhere my model is consuming gas.

I know it's a closed model because I've got fluid assemblies that show the interior and exterior are separated so unless someone changed the rules Flow should only allow gas to enter or exit the model at BC's correct?!?!?

IDEAS?!?!?!