Nothing’s stopping a driver from getting out of their car and walking away. So never.
If I leave my car somewhere because a group of people gave me no choice but to do so, yes it is stolen.
Are you planning to do this? Do I need to contact the authorities?
No. It would be robbery if they stole your car. If you had to leave your vehicle and walk away, and your vehicle was subsequently towed, you might be able to sue them in court for the cost of towing and impounding your vehicle and maybe something for your time. But never would it arise to kidnapping or theft as a criminal matter. You are not your vehicle. It is only kidnapping or false imprisonment if you, your own physical person, are prohibited from leaving.