Interesting point about the tracking device. Hell, it's been pointed out in discussion before how Leia was pretty damned stupid to lead the Empire straight to the Rebel Base immediately after lying (with her home planet and the lives of billions at stake) about its location. So I guess that whole scenario could have been thought through a little more. But I guess there was time between their discovery in the detention block to the time they got back. Tarkin and Vader are even alerted that there is a disturbance in the detention block, to which Vader replies, "Obi-Wan is here!" and then leaves the room. Tarkin could have after... but wait, in the dialogue after the Falcon leaves, Tarkin makes it sound like this is Vader's idea! And like I said, he left to find Obi-Wan immediately after having found out there was someone on board the ship, so, yeah, I have no idea when that could have happened!