That's also assuming it was the local garrison that got to them. Vader was the one who deployed the sandtroopers and we all know how extra he is.
And frankly if it was Vader's troops then given how highly sensitive that data is, they also don't know if anyone has seen anything as well as Luke and the droids are gone. Meaning they could very easily have been making off to the nearest space port (which they do). Even if they saw it and didn't have the plans if anyone got to them you could get some info off of that meaning that's still a loose end. And better yet should any Rebel be coming to their aid you also have tied up loose ends with the murders meaning they can't find out what they saw if anything and you can quash anyone else reaching the plans before you do since the leads are dead.