During ANH, Han, Chewbacca, Luke, Obi-Wan and the two droids have left Tatooine aboard the Falcon just in front of the search troops (they fled under blaster blows).
At this point, the Empire know that the "two droids they are looking for" are on board the Falcon.
Did the Empire know the Falcon was going to Alderaan, right after the escape from Tatooine?
When the Falcon arrived near Alderaan, a TIE fighter attacked them, then they followed it right to the Death Star. Did they stop all the ships that went on the planet after it exploded?
Or did the empire kidnap the Falcon because he knew the droids were on board? Was the empire just "lucky" to catch the ship that contained precisely what it was looking for (the plans of the Death Star)?