And yet, as seen in the original Star Wars, Vader's ship immediately finds Leia approaching Tatooine, which we learn is the galactic equivalent of an abandoned outlet mall. How the hell? In every other movie, when someone goes to light speed, they can't be followed. It's as if someone on board transmitted the info on their location. Now factor in that one of the droids on the ship was built by the guy who's chasing them ...
"Yay, almost done! Now to put a GPS in him in case I ever turn into a child-murdering dickwad."
Of course, we're not suggesting Threepio is doing this intentionally -- that would be altering his character from merely a golden D-bag to a full-blown villain. But what if Vader figured out that his childhood robo-pal was hanging out with the Rebels, and found a way to monitor his activities remotely? That would explain how they tracked Leia's ship. Also, how did the Emperor knew the Rebels exact plan in Return Of The Jedi?
The official novelization's explanation is "He read the official novelization."