I did a job a few years back for a reg client. Cleaned the lounge carpet, client walked into lounge shut the blinds and went on hol for weekend. When client returned from holiday, the carpet had light areas, that looked like the colour had been removed, (this was also comfirmed to the client by the carpet salesman that lived opposite, twat). The light areas were where the client had walked across the wet carpet and when it dried, there was a light reflection on the pile. A re-clean and client happy and carried on using me.
This looks similar, due to random colour changes ie. not from a re-appearing stain, traffic lanes etc
Reclean, set pile and dry with air mover, before you leave or let anyone else on carpet