As a professional carpet cleaner in Tampa FL I see all types of carpet stains, and most of the time I have the luxury of knowing what type of stain I'm trying to get rid of. Then, sometimes I get an unknown carpet stain.
A customer in Tampa FL had a rental property where the tenant had not bothered to take care of the house and left the carpet in pretty bad shape. After the tenant was evicted the landlord found several dark, oddly shaped stains in the carpet. This photo is one of those dark stains. The customer had no idea what was spilled and left to sit in the carpet. He was considering getting rid of the carpet all together and laying down hard wood floors.
Because the stain could not be identified by sight, touch , or smell (taste was out of the question) I tested a couple of different solutions to see what would work best. After a couple of tries I started to get some good results with one of our more aggressive pre-sprays. After spraying the stains I let them sit for 15 mins so the stains would loosen their hold on the carpet. Then I used Chem-Dry's hot carbonated extraction process to remove the unknown carpet stains.
My customer was so impressed he said he was "tickled..." to see the stains were gone. He said "Some days are better than others and this is a good day." In the end I was able to rid his carpets of the unknown stains, saving him time and money on his investment property.