On a couple of occasions, I have noticed this effect when my car is parked under high voltage power lines (quite safely, no danger of contact).
There is a very small amount of capacitance between the car and the power line, and the car is now at some voltage above ground, due to the electric field between the power line and the ground,
When you touch the car, a very small current flows through you to ground.
If your car is not near any power lines, I do not see why you are seeing AC volts.
Intriguing.
JimB