The anti-thief device was a protective mechanism the Third Doctor incorporated into Bessie to keep her from being stolen. It was activated by flipping a small switch on the car's dashboard next to a red placard entitled "ANTI THIEF DEVICE". This would generate a powerful force field around Bessie that pinned any potential carjackers to the vehicle who had made physical contact with her and kept them from breaking free until the device shut itself off, presumably to leave would-be thieves trapped and eventually apprehended. It could also be used to temporarily incapacitate individuals as a means of purely detaining them. It appeared to be magnetic and linked to a person's natural electric supply in the body, which could be used as a tether to bind the individual to the magnetic field, at least in the case of humans.
The Doctor used the anti-thief device against a group of criminals who had stolen a large freighter carrying the Recovery 7 landing capsule. The Doctor posed as an elderly bystander with his car broken down on the road, prompting the men in the truck cab to get out and help him push. This gave the Doctor a chance to set off the anti-thief device and pin them to Bessie, allowing him to reclaim the Recovery 7 capsule. When UNIT recovered Bessie, the Doctor asked if they found "two angry men" attached to the car, but Brigadier Lethbridge-Stewart claimed his men found just the car. The Doctor remarked that the power of the force field wasn't strong enough yet if the men were able to escape so quickly. (TV: The Ambassadors of Death)