Presuming its a modern car (and if we are talking about keyless entry/start we are), well then you just plug an "Emergency Start Device" into the OBD port and drive away. Heck a lot of these "Emergency Start Devices" can also unlock the car, but often involve pulling panels/lights from the car to get to the can bus to run the attack.

So that attack when done on its own is mainly left to stealing cars off drives at night rather than say from a supermarkets car park during the day.