Could be wicking or it could be an unbalance of the PH level.  When people or cleaners use a soap based cleaner that concentrates in one area, this can cause a high level of PH. This condition causes a reaction in the carpet known as browning.  Try a mixture of water and 20% vinegar and lightly spray the spot, then lightly wipe with a towel on the effected spot.  It this helps, let dry, then apply again until the browning goes away (PH balance is in lowered)...Frank, Heaven's Best Carpet Cleaning.