Welcome to the forum, if you could add your locaton to your profile, it would really help us to figure out such issues - thanks :)
It does surely sound like abscounding, I must say this happened to me a few years ago and I actually caught the buggers flying off to the great blue yonder.
Abscounding does differ from normal swarm activity, at least was the case I noted. It seemed the bees HAD a place to go already picked out and JUST LEFT. Where in swarming, about half the population evacuate the hive and take temporary roost until scout bees find a suitable new home.
I suspect sence abscounding typically happens during a short warm spell during Winter (at least very late Fall) the bees NEED to have a new home picked out prior to leaving or the very well could be committing mass suicide if caught on a branch with no protection against the wind and cold.
Good luck, I hope you find your answers and again welcome to the forums.