Worked with a company that had an issue with abnormal fuel burn on an MD11F. Several months passed before the aircraft was being re-positioned empty and the flight mechanic went back for a smoke noticing a vapor stream out of an over wing fuel panel, panel was loose and would suck out 10K lbs from top off on average (the MD11 auto transfers fuel). Unique yet simple example as the first thought was an engine issue that could not be pinned down.