This actually happened to some folks I know. I haven't spoken to them personally about this, but will give a few facts as I know them, and the outcome.
They are an older couple, driving a nice car and stopped at a convenience store in a nice part of town. The wife waited outside in the car, while the husband went inside the store. The wife did not lock the car doors after her husband left. While she's sitting in the passenger side, a strange female opens the drivers door, gets in and attempts to back out. The wife fights with the strange woman. At some point, the husband comes out of the store and the strange female points a gun at him, in the meantime the wife jumps out of her car. The strange woman left in the older couples car.
Several police agencies quickly got involved, chase begins, spike strips deployed, car totaled, strange woman in custody.
The strange woman- Apparently she had been driving a stolen vehicle and felt like it was time to ditch it for a different vehicle. I'm guessing the police found the vehicle she was driving at the convenience store and it was registered to someone in another city about an hour away. As it turns out the strange woman had previously been to a casino and had followed a lone elderly woman out of the casino and car jacked her and her car. The strange woman murdered the casino woman and took off in her car. It's now obvious the older couple were being carjacked by a desperate murderer.
What-if scenario #1- Let's say you're the husband coming out of the convenience store (armed IWB), and you see a strange woman in your drivers seat with your distressed wife inside and the strange woman is attempting to leave, armed and pointing a gun at you. I know there is not any one definitive answer to this because too many variables exist that we don't know about, but it would be a very tough situation to be in.
What-if scenario #2- Let's say you're the wife sitting in the passenger seat and you had your doors locked. You hear someone attempting to open the drivers side door handle and you look at each other at the same time and you see she has a gun pointed at you, yelling to unlock the door?
Outcome- Lesson learned for the older couple, she admits that she was "perfect prey". The wife baled, the strange woman left in their car, the couple was physically unharmed.

