This is a naturally occuring post-cleaning situation known as reappearing or wicking stains. It’s caused by staining matter from a prior spillage having dried down at the base of the carpet pile. So after cleaning, you may have had the visible area looking spot-free but the old problem was still lurking at the bottom – now free to roam by moistening it.
Once the stain is wetted by a thorough cleaning it becomes mobile and wicks up the fiber to the top. Since wall-to-wall carpet dries from the bottom up, the top is the last to dry completely. So, this is how the “invisible” stain suddenly appears.
You can take this follow-up step yourself to remove most or all of this reappearing stain: just moisten it and blot with an absorbent white towel. The best way is to mist it, cover it with toweling add a layer of aluminum foil or wax paper on top of the towel and then put something heavy on top like a telephone book. Wait about a half an hour and then remove. Repeat as needed.
An alternative is to use a carpet spot remover solution or drycleaning fluid type spot remover. Make sure they’re safe on your carpeting first; try a test area in an out of sight place. You can always rinse lightly and do the blotting step mentioned above.
If you prefer a dry-extraction, carpet cleaning compound, lightly massage or brush the compound into the stain. Leave it to work for 30-60 minutes to absorb the stain, then vacuum.
If you have any doubts about trying to remove post-cleaning stains yourself be sure to contact Ayoub N&H Carpet & Rugs for powerful, professional carpet cleaning in MD, DC, and Northern VA and get the best possible results.