Charles Henderson sold his car several years ago, but he still knows exactly where it is, and can control it from his phone.

[...] Manufacturers create apps to control smart cars — you can use your phone to unlock the car, honk the horn and find out the exact location of your vehicle. Henderson removed his personal information from services in the car before selling it back to the dealership, but he was still able to control the car through a mobile app for years.

That's because only the dealership that originally sold the car can see who has access and manually remove someone from the app. A full factory reset of the vehicle doesn't revoke mobile access, Henderson said. In order to revoke app access, you should go to a factory-authorized car dealership.

On smartphones, a factory reset wipes all the local data off the device so you can sell it to someone else. So-called internet of things devices store information in servers far away from the actual hardware. This means executing a factory reset on your car only resets the car — the data still exists in the cloud for other people to access.