Bingowings said:

Han accidentally finding himself in a system with one inhabited planet, the main city of which is being run by the guy who previously owned the ship he is sitting in, is the sort of galaxy shrinking coincidence that Anchorhead doesn't like and he likes bees.

Maybe the fleet travelled quite a distance with the Falcon sitting on the back of The Avenger (without anyone else in the fleet noticing it)?

Drones of the Empire don't get paid to look out the window. There doesn't seem to be any windows on the back end of a Star Destroyer either. ;)