Peter_I, I think both are correct. It just depends on the vehicle and the conditions.
I've had cars where I'm sure it is the vehicle building the charge. I can touch metal inside, nothing happens. I slide around without getting out, I can touch metal and nothing happens. But if I get out with a hand on metal I can feel a big Snap through my shoe to the ground. And if I get out without touching metal, I don't get zapped until I touch the car.
Other vehicles, I think it is me sliding across the seat, because I won't get shocked if I touch the keys while still inside, but if I start sliding around and -then- touch metal, I get a spark while still inside. If I get out without touching metal, sometimes I can detect a spark through my foot to the ground, and another spark when I touch the car. That would seem to indicate I have the charge.