Swarm just flew away today - now what?

Sounds like absconding behaviour. If it was you will have a basically empty hive with brood left behind.
If it was a swarm then they are most likely done for a while. Crowding is the cause of swarming 99.9 % of the time so be sure your other colonies have room.

Go into the colonies and look for ones with capped queen cells and no eggs. The hive that swarmed should fit that description. Reduce the number of queen cells left to one or two to reduce the possibility of more after swarms. As mentioned you may find other colonies with cells started are getting ready to swarm as well. Once they decide swarming is in order simply giving them more room is likely too late.

In the spring I almost lost a good swarm that way from a hive that had become honey bound. I saw them go about 100 feet high into a tree. Knowing that they don't always go to a permanent home right away I hung a swarm trap and they entered it within a day. If you have swarm traps out you don't always lose the swarms.
It has been my best colony and made it to 5 boxes by August.

Always keep a swarm trap nearby, even if it isn't hanging yet.