I've worked with a number of interesting people during the course of my working life, including some that were downright quirky. RM was one of the more quirky individuals from those days.
My workspace was a cubicle, one wall of which consisted of file cabinets. The other walls were movable panels, approximately five feet in height, with an open space for an entrance.
RM was in management, and I was an accounting clerk. I don't even remember how this got started; but, when RM would come to my cubicle, whether with a question about his expense report or just to chat, he would stop at the cubicle entrance and knock on an imaginary door.
One time, after I had granted him admission, he was sitting in the chair next to my desk, with his leg stretched out at an awkward angle. I asked him why he was sitting like that, and he replied that he was holding the door closed so that we could talk privately.
On another occasion, he knocked on the imaginary door; and I mouthed the words, "We're closed."
In reply, he mouthed the words, "Then you should turn the sign around."
Thanks to Hilary at The Smitten Image for including this post as a Post of the Week.