"No" what?

I was walking into the public toilet with Ah Teo one weekday morning after a breakfast at Crown Tower, when I spotted a peculiar sign on top of the emergency light. It was just a blank NO sign with nothing in the middle of it. Usually if it is a no smoking sign, there would be a cigarette bud symbol in the middle, or no food; a burger and a drink. But this one, there's nothing in it o_o..

I have taken the liberty of thinking and listing a bunch of possible things that the "No" sign is for, which they probably ran out of budget and did not get to finish it. It was in the public toilet, so that would narrow my list down to a few possibilities on what could be the "No" sign was for.

If you have any other ideas on what the "No" sign was probably for, please feel free to share with us =)