It happens sometimes when a blood vessel breaks during laying. It's usually not an indication of a problem. I would just check vents to make sure there is not blood on any of them, that could draw pecking and cause injury. Happens more often to new layers, but can happen to any of them occasionally, particularly if they lay a larger than usual egg. As long as everyone is acting normally I wouldn't worry. If it starts happening a lot, I'd try to figure out which bird was doing it, but you may not see it again.