The only mating swarm I believe I saw was from a full-sized hive - they flew and then came back the queen was laying shortly afterwards. Can't remember the type of queen - my current ones tend to be quite orangy in colour - although there's quite a variability.

Mini-nucs do abscond AFTER the queen has started to lay - often just a few days and before you get around to putting a queen excluder over the entrance. It does seem to be a heat thing and the mini-nucs don't have to be in full sun either - just a decent warm day. I've never seen it from 3 or 5 frame nucs using std BS frames but have seen it from some small plywood mini-nucs I have.