Meaning and origin - "More on point than a sock"

In TV Show "Elementary" - Season 2 - Chapter 15, Dr Watson starts saying:

More on point than a sock

What is the meaning of this expression? What is the relationships with "points" and "socks"?

Context:

Corpse de Ballet

It’s an average morning at the brownstone, which means Sherlock has an entirely consensual, entirely independent ladyfriend over and has politely informed Joan of this fact via post-it, which Joan considers more “on point” than a sock on the handle. (It’s a ballet pun, like the title. The writers are having fun this episode.) After politely offering their guest coffee, Joan and Sherlock are called to a murder investigation– Nell Solange, a ballet dancer, was cut in two by a wire in the theater rafters.

If @Kreiri is correct, than "sock" refers to hanging a sock on the outside door knob of your dorm room to warn your roommate not to enter because you are inside having sex. Go here. The explicit sign on the door is more to the point than the symbolic sock.