Really

 The car looked as if it might be about to back out onto the road in front of us so we slowed down. We both saw the driver- only we didn't. As we went by we realised we'd been fooled by the headrest that was sticking up above the driver's seat, 

Odd thing is I could give you a clear description of the person I thought I saw. She was in her late thirties/early forties, tall, slim, upright, with a strong, bony face and dark blonde hair pulled back into a pony-tail.

And I derived all that from a headrest?  Really?