Weird Stain unknown origin

Hey all,

This is my first post, but I figured this forum would be the best to answer my question.  I had my sofa against an exterior wall that has the garage on the other side.  My wife and I have since moved, but when we pulled the sofa away from the wall, there is ~1inch thick strip of black carpeting that runs against the wall almost the whole length of the sofa when it was a light beige brown originally.  I've heard of black mold, but when I picked up the carpet away from the wall, neither the tack strip or the backing of the carpet has any discoloration.  I'm stumped and don't want to have to replace my carpet as it's only 5 years old and in great shape otherwise.  I'm at a loss as to what it can be.  I've tried my little green steam machine, to no avail.  Does anyone have any idea what it could be?  I'll get it taken care of professionally if it doesn't require replacing my carpet.

Thanks,

Ray

What does FS mean?

Very common!

Edited by Hammy - 12/June/2008 at 11:24pm

try a citrus gel .sometimes it works.

I've had good results with Traffic Slam, but if I can find something where I don't have to scrub as long I'll switch.