I took the roof off Hive E today. This is the late swarm from 2016 which is housed in a small derelict greenhouse.
I had put an insulating box between the brood box and the roof. I'd seen ants going up and down the hive. What I hadn't expected was that the ants had set up a nest in the insulated roof. There were lots of black ants, some eggs and a few immature grubs.
I can see now that the insulated roof would be ideal for an ant nest. Defensible; warm, well ventilated and with an immense food store nearby.