I’ll have to remember to ask a buddy about this. We were traveling, and I think it was an “In-N-Out” stand that we stopped in at. He was bragging about growing up near one, and if you ask for it, they will make you a “secret” burger of some sort. But it’s not on the menu. Of course when he asked the gal at the one we were at she looked at him like he had two heads.
I imagine where he grew up the chef was a friend of an older brother or something and made up the “secret” burgers for them.