[unwind]

That's a really interesting idea, that had never occured to me.

Probably because I'm pretty aware of the close-doors button actually looking like >|< on typical elevators. The vertical line, I guess, represents the opening.

See for instance https://www.google.com/search?q=elevator+close+button&tbm=is....

[rodh]

Actually there are buttons with a line through them, and ones without. They maddeningly mean different things. See http://petesguide.com/symbols/elevator-button-symbols/