Carpets can hold onto residues that allow them to collect soil. These pictures are from a recent carpet cleaning in Tampa FL where the home owner had a maid service come in and clean her home. A few days later this dark spot started to show up in the entryway of her bedroom. The area smelled like cleaning products so she called us to come take a look at it.
Carpet cleaners run into this all the time. The spot was caused by a house hold cleaning agent that was spilled on the carpet. It smelled a lot like Windex or another glass/hard surface cleaner. The problem with this type of carpet spot is it will keep returning no matter how many times it's cleaned. The powerful soaps and surfactants stay on the carpet fibers as a sticky residue. They attract soil and dirt, so the more she walked over the spot the more dirt it would transfer to the carpet. That's why the spot kept getting darker and darker over time. Normal carpet cleaners and spot removers use a lot of soaps and if they are not rinsed out good enough the spot will return in a few days.
Sometimes specialized treatments are needed to take care of tough reoccurring spots in your carpet. We used a specialized carpet cleaner that encapsulates invisible residues so they cannot attract soil to the carpet fibers. After cleaning and encapsulating the spot was gone never to return. Over time the encapsulating carpet cleaner will slough off taking the dirt attracting residues with it. All is well that ends well.