I've had it happen once out of the hundreds of these I've done. The menu board at this one location is set up so that the cashier can see the driver. I thought I was being discreet by hiding the camera, but she saw me.
When I reached the window, she questioned, "What you taking pictures for?!"
I made up a quick story, "Oh, I just like to post on Twitter where I'm eating at."
She started laughing at me. I'm pretty sure she actually bought it.