Does it itch?
What breed is he?
Has he had skin issues?
Do you notice these marks anywhere else?
Does it appear inflammed or raised from the skin?
Is it on his sternum only?
Any history of skin issues or allergies?
Any chance he could have gotten into anything like rat poison?
What color are his gums?
Do you see any unusal marks on the gums or inside the ears (pinna)?
How old is he?
It would be helpful if you could respond to the rest of the questions. This could either be a bruise or simple be a change in pigment. If he was scratching at it then this would explain the red streaks. This may be an area where he applies a lot of pressure when laying down. Since these guys are so low to the ground everything rubs on their chest.
It is also possible that this is a bruise and if you notice bruising anywhere else on the body or he has pale gums or bruising on his gums or ears then he needs immediate veterinary attention. If this is the only area then it is possible he hit his chest on something.
Chances are this is just pigment change and some scratching he may be doing as a result of allergies. But do keep an eye on him for further changes if they occur.