From a practical point of view, I would imagine that the instructor signs the authorization for both flights (to and from the examiner airfield) as one document. I mean, you don't sign/carry three separate forms for the three legs of the QXC, do you? But as said, I would add the condition (and put it in writing) that the student phones the instructor before flying home.
In my case, I had to fly to an airfield around 12 minutes away, meet the examiner there, carry out my skills test, drop off the examiner and then return to my home base. I had three PLOGs, one for the flight there, duly signed by my instructor, one for the skills test and one for the return flight, again signed by the instructor and issued just in case I failed the test. I passed, returned to my home base.
A few weeks later there was an article in a GA mag about the legal and insurance ramifications of PPLs flying back from their skills test, having passed their exams but without actually possessing the licence. The human factors side of it wasn't mentioned, it focussed purely on the legal and insurance side. Since then, school's rules state that the instructor has to fly with his student to the examiner or the examiner has to come with the student....