I think the idea of alarm pheromone is a good one.  However, I'm going to
suggest that there may also be other pheromones involved.  I know the smell
of alarm, and there's also a sweet honey smell that is different.  (I have
a very developed sense of smell---when I was doing field botany, I would
often i.d. plants simply by smelling them.)
 
As I said before, it might be a good idea to check what's going on in your
hive, when you get these smells---swarming behavior?  Something disturbing
the hive?  Something else?
 
Jane B.