In The Empire Strikes Back, when Vader's Star Destroyer is pursuing the Millennium Falcon and it suddenly disappears, all of a sudden there is a bunch of bounty hunters lined up waiting to do Vader's bidding. Where did they all come from?

The pursuit is happening in the middle of nowhere and even if the Empire maintains some kind of bounty hunter alert system, it would take a while for the hunters to get the signal, not be preoccupied with some other hunt, plot a course (avoiding the nearby asteroid field), dock with the Star Destroyer and make their way to the bridge. By this time, if the Falcon has escaped they really would be 'on the other side of the galaxy by now'.

Alternatively, does the Empire (or Vader) retain a group of bounty hunters that hitch a ride on Star Destroyers waiting for some mission too difficult or dangerous to send their own troops on?