I just received the following link from Arnia showing various parameters which can change just before and during swarming from a hive. I thought it was quite interesting, and although the signals are probably too late to take pre-emptive measures, at least one would understand why the hive weight dropped. There is also a nice short video of some swarm-related worker behavior on the frames inside the hive before they leave.
http://www.arnia.co.uk/the-behaviour-of-honey-bees-preparing-to-swarm/
Enjoy! 