I've been watching Star Wars for years, and I just caught these lines:
INT. DEATH STAR - CONFERENCE ROOM Tarkin pushes a button and responds to the intercom buzz. TARKIN Yes. VOICE (over intercom) We've captured a freighter entering the remains of the Alderaan system. It's markings match those of a ship that blasted its way out of Mos Eisley. VADER They must be trying to return the stolen plans to the princess. She may yet be of some use to us.
What? Why would someone with the only copy of the stolen plans take them back to the Death Star, to give them to the princess, who is in detention there? He didn't say they may be trying to rescue her, or that they were probably destined for Alderaan.
The question is not about Luke/Obi-Wan's motivations, or why Vader let them 'escape' later. My specific question is:
Why does Vader assume 'they' must be trying to return the plans to the princess?
Anyone who has now come in possession of the plans must know the princess was captured, that's why/how that person now has the plans.