<begin ramble>
The hawk may have been scouting for mice around your yard and coop. I understand that some people have problems with hawks but their are many ways to deter them without killing them. They mostly eat pest species (rodents), chickens tend to be a difficult meal. Predators nearly always choose the "prey of least resistance." In other words, they choose the easiest meal ticket, because the wasted energy trying to kill a large healthy meal (aka a chicken) could mean starvation if it is a failed attempt.
Hanging cds around your yard is favored by many people. You could also add a turkey to your flock. Search the forums, their are many stories of turkeys flying to attack hawks. Some hawks aren't afraid of dogs most likely because it is an older more experienced hawk that knows the dog cannot reach him. A 20 pound turkey flying towards them will definitely make them think twice. If you go this route go for a smaller heritage turkey such as a Bourbon Red or blue Slate. They are large enough to handle themselves but still capable of flight. Some people attract crows to their property, because crows chase off hawks as do most song birds. They gang up on hawks to drive them out of the area. A paintball gun is also effective to startle the hawk away form your birds. Good Luck.
</end ramble>