Hi chris
some years ago Iwas called to a simillar problem.It was a wooven wool carpet and what had happened
was that during the manufacturing process oil from the looms had periodically dripped onto the carpet leaving a residue which of course did`nt show up until it had been fitted and walked on for a time.The telltale signs for this problem was that the marks were in a straight line.Sprinkle finedust from your hoover over the area,walk on it once,then vacume thoroughly,you`ll be left with clearly defined residue
patches which you can then tackle.Then again the spots could be all over the place,in which case this has been a complete waste of time,but

regards rice