Should I be concerned about dark spot under veneer? (Photo)

I had procedure for veneer on my two upper Lateral Incisor teeth around 4 years ago. I noticed that when I flashed my tooth with phone flashlight, I had a small dark spot. I wasn’t sure if it’s a tooth decay or a leak in the bond of the veneer. Therefore, I wanted some advice as to what it maybe and should I be worried. So far there has been no major pain from tooth. It’s been just sensitivity from hot/cold and itvaries/not frequent.