If you move a swarm trap at night, you will get all the field bees in the box which is a good thing. As long as you have the time and are not prone to fall down in the woods at night, it seems like a good idea to me to collect the box at dusk. I would wait until daylight to actually move the bees from the retrieved trap to your permanent hive. It is much easier to get bees from one box to the next in the daylight!
If you see more than about 20 bees going in a trap per minute and some bees are carrying pollen that is a very good sign! Time to check the trap. A typical swarm is probably about 10000 bees (I would guess). If you have 10,000 bees in a box, you are going to see a consistent flow of bees in and out. Incoming bees will be bring in pollen and nectar. The bees will not be circling the trap at that point, they will be darting in and out on a mission, like a normal hive.
I missed a beautiful big swarm today. 20th swarm this season. A double basketball sized swarm. By the time I got my stuff and got out there, it had taken off :( Pretty sure that swarm came from my Jumbo frame hive of Carniolean bees. Those big frames throw some huge swarms.