Why do my porcelain veneers have grey spots? (Photo)

I've had vaneers on my six front teeth for the past 7 years. I think they look nice and natural and I've never feel too conscious about them as the dentist put some 'natural touches' on them. However over the past few weeks I have started to notice grey patches on my front two teech (particular on the left front tooth when looking at the photo). Does anyone have any idea what this might be caused by. There is no sign that my tooth below is rotting or anything! Thank you! Rose