During a sunny spell today I checked the entrances of all hives for drones (I have yet to inspect inside hives at all, and am paranoid about swarms).
Just one hive (a 6-over-6 frame double nuc) had drones, and it had quite a lot of them. This scared me into doing a first inspection on that colony.
I took the opportunity to move them into a full sized hive.
No swarm cells, thankfully. Quite a bit of brood. About 10 frames of bees (bees not brood). Not that much honey, so feeding them now.
Slightly more relaxed on the other colonies now that there were no swarm cells (or even cups) in the one colony that was producing drones.