Always, always, always take the keys out of the ignition when swapping drivers!
A similar type of theft happened near me in the early '00's at a prestige garage when they were on a test drive with a 'customer' in a demonstrator & the salesperson and customer stopped to switch over. Customer slid across as the employee stepped out of the car, and then took off with the car...
SGT was the garage - selling Alfas, Morgans,, Lotus, Mitsubishi's, & Subaru's - think the car was a Scooby that was ultimately used in a ram raid.
Be aware when selling something tasty, that's for sure...