Line in your veneers possibly the line of cement
It is possible that the line you are seeing is the interface where the tooth and the veneer meet.
There are many contributing factors that could cause this. Some include:
Improper preparation of the teeth so that this interface is visual to the eye.
The cement line is excessively thick due to poor fit of the veneer.
The cement could have taken up stain.
Any single factor or combination of the factors could cause a possible line.