As far as I am aware no-one has run a control measure of outside air all the measurements have been made with a degree of 'confirmation bias' so measures are taken within the cabin as the 'only source' of oily fumes could be from the compressor. And of course swabbing the seats will find traces of the oil contaminants in any aircraft that has taxied up behind another.
Just walking around an apron or down the jetway, smelling the kerosene and other fumes, it becomes obvious that there are a multitude of unmeasured sources. Including, sitting in the departure queue ingesting exhaust from the aircraft ahead and 'conditioning' the cabin air with that. Doesn't matter how good your seals are in that case.