Barry,
It sounds like the customer got there first with a bowl of water (+ goodness knows what else) and kindly washed some into the underlay. Posibly the substrate may still have been damp when you cleaned.
You have removed the surface contamination and the remainder has wicked up to the surface.
Re-treat the stain with something like Prochem B144 Stainpro, agitate and extract with a hand tool, drying the carpet as much as possible. To be on the safe side, either pad-dry or (safer on this particular carpet) leave a weighted terry towel to absorb any further wicking.
John.
