Thanks everybody!
I spent the last hour filling "obvious" places where a dog could get in. I read stories about dogs that can break through pretty strong wire, but (maybe I'm naive) I've never seen any dogs around here that seem to be so inclined. In fact, in 5 years of having our hens roam safe, this is the first dog that came into our yard. Hopefully just a statistical outlier and not a developing trend.
The bad news: While I was out fixing holes in fences I thought I knew the house to which the dog had retreated (where he was barking). Turned out it was a very white yellow lab behind bushes that just resembled the attacking dog. Now I don't know where the dog came from / through or where it lives. I could start canvassing the neighborhood and asking around, but I'll probably wait until / if I see it again.
Do dogs tend to get a "taste" for killing / eating chickens like coons do?