It was our fault.
We had left the chicken coop open - we try and shut it around dusk but sometimes forget until later. Its one of those shared responsibilities which, in the absence of any visible raccoons, slips into the after-the-kids-are-in-bed-turmoil and then, sometimes, later.
This morning the only signs of the invasion are some feathers and a large boulder against part of the coop where the foundation of the chicken coop is a bit weak. I can see that part of the bird netting over the run (which is supposed to stop the chickens flying out over the fence) is pulled down and is, presumably, where the raccoon climbed down off the fence. Bird netting is not going to stop a raccoon but we shall have to ensure that the coop itself can withstand a more direct attack now that the raccoons have them on their radar.
Edited to add:
Josh says that they first got in when the door was open. He rushed out when he heard them shouting. There were two raccoons, one in the coop and one outside. The chickens were flapping around and pucking like mad. He hit one raccoon with a rake and, displaying his pastoral roots, vowed he would have shot them if he had a rifle. This is why we not have a rifle.
He explained that no animals mess with our chickens.
Then, at 3am, the chickens started up again. Josh went outside and found the raccoons had returned. This time they had removed a cinder block which is part of the foundation of the coop. It is a very heavy cinder block and was partially buried so this took persistance. Josh moved a large boulder to reinforce this area and has been out there checking on it this morning.
In another development, this morning I removed all the eggs from the henhouse by mistake. you are supposed to leave ONE egg so the chickens continue to lay in their nesting box. Apparently they count in a binary manner - Egg 0 or Egg Some. If you take all the eggs the chooks will relocate the nest to a safer spot. The chickens were very agitated when they went to lay and found the eggs gone! We replaced a few eggs and they settled down.
The chickens' intelligence has grown in our estimation as well as their ability to communicate.