Welcome to Beesource, Rah! Sorry the bees are gone.

 

possibly an "abscond" from a highly infested colony that had lots of mites, small hive beetles or wax moths.

 

I can't say I've seen much of that happening over the years. (None?) Or maybe I'm just a good beekeeper and don't often allow hives to get to those states.

 

I have seen if the queen is not with the swarm in time they go back to the original hive.
or if they changed their mind they could have just left, mouse pee, fuel smells, vibration etc can have bees leave.