It’s not uncommon to have home buyers see dark staining in homes and have concern that it may be mold. There is an occurrence not known to most home buyers called “ghosting” that can cause dark stains in many areas of a home.
Indoor air contains different types of particles from both indoor and outdoor sources. These particles will eventually land on interior surfaces through a variety of ways. When they build up, they cause a staining (can appear on walls, floors, ceilings, appliances, etc).
This can happen due to smokers in the house, gas burning appliances in the house, candle use or just pressure differentials in walls that cause the carpet to act as a filter as air passes through it.
To read more in depth, go to the web site link below.
http://www.ct.gov/dph/lib/dph/environmental_health/eoha/pdf/technical_brief_ghosting.pdf