Color change on t-shirt only noticable under natural light. What is the cause of this?

I have a powder blue pittsburgh penguins t-shirt. The other day while ironing it I notice in a few locations on the shirt where the color is of a lighter shade than the rest of the shirt. They are not spots since the size and shapes vary and I do not use bleach. The strange thing is the discoloration can only be seen in natural light like sunlight. You can't even tell its there when inside a room where its only lite by light bulbs. Its strange how this only appears in sun light. Has this happened to anyone else and can you explain how to fix if you can?Color change on t-shirt only noticable under natural light. What is the cause of this?You can't really fix this. It sounds like a dye accident on their end. Or detergent sat on it too long without agitating in the washer.