12 year old veneers lightening near gum line of canine tooth. What can this be? (photo)

I have had veneers on my top front 6 teeth for about 12 years. The right canine has started lightening near the gum line. There is no difference in the "feel" of the veneer there. I've heard of veneers "darkening" because of damage to the tooth below or a leak, but lightening? what could this be? I only noticed this recently. In the picture look at the oval shaped whiter area at the top of the tooth. Thanks in advance!